The story of Umm Qirfa comes up frequently by critics to show how Mohammad and his companions were barbaric for killing a woman as claimed by them for merely being ‘a noble woman of high rank who did nothing wrong other than adhere to her traditional beliefs’.
It is portrayed by critics that she was killed because of her faith and secondly being a woman of high rank. And it’s also claimed that the Prophet (p) raided her tribe for no reason. Let’s test these claims.
Let’s read two accounts:
THE EXPEDITION OF ZAYD B. HARITHA TO UMM QIRFA
Abu Abdullah Muhammad b. Umar al-Waqidi related to me that Abdullah b. Ja’far related to us from Abdullah b. al-Husayn b.al-Husayn b. Ali b. Abi Talib, who said: Zayd b. Haritha set out with merchants to al-Sham. He had money from the companions of the Prophet, and he took two testicles from a billygoat, tanned them, and put the goods in them.
Then he set out until when he was outside Wadi al-Qura with people from his companions, groups from the Banu Fazara of the Banu Badr saw him and struck him and his companions until they thought they were dead, and took what Zayd had brought with him. But, Zayd escaped and arrived in Medina before the Prophet.
The Prophet sent him on an expedition, saying, ‘Hide by day and march by night.’ One of their guides set out with them. The banu Badr were about them, so they looked for them in the morning from the top of the mountain overlooking the road, which they believed they were coming from. Observing the determined route by day, he says, ‘Move freely, there will be no harm to you on this night of yours.’
When Zayd b. Haritha and his companions were marching to the route at night, the guide mistook the road with them, and he took them on another road until in the evening they knew they were mistaken. They they waited for them in the night until it dawned on them, but Zayd b. Haritha stopped them as they were far from their goal.
He said: then he advised them not to disperse and asad, ‘when I proclaim takbir, you proclaim takbir.’ They encircled the residents, then he proclaimed takbir and they proclaimed takbir. Salama b. al-Akwa set out seeking a man of theirs to kill. He was thorough in his search, and he captured a girl, the daughter of Malik b. Hudhayfa b. Badr, whom he found in one of their houses-her mother was Umm Qirfa. Umm Qirfa was Fatima, daughter of Rabi’I b. Zayd-and they plundered. 
Kitab Al-tabaqat Al-Kabir – Ibn Sa’d:
Then (occurred) the sariyyah of Zayd Ibn Harithah against Umm Qirfah in the vicinity of Wadi al-Qura, which is at a distance of seven nights journey from al-Madinah, in the month of Ramadan of the sixth year from hijrah of the Apostle of Allah, They (narrators) said: Zayd Ibn Harithah set out with merchandise to Syria. He had with him goods belonging to the Companions of the Prophet.
When he was near Wadi al-Qura, some men of the Fazarah, a branch of Banu Badr confronted him, beat him and his companions, and robbed what they had with them. Then Zayd recovered (from his wounds) and arrived before the Apostle of Allah, and informed him (about it). The Apostle of Allah sent him back against them. They remainedin concealement in the dayand travelled in the night.
Banu Badr became aware of them. Then in the morning Zayd and his Companions said takbir (Allah is Great), surrounded those who were present there, and captured Umm Qirfah, whose name was Fatimah Bint Rabi’ah Ibn Badr and her daughter al-Jariyah Bint Mailk Ibn Hudhyafah Ibn Badr. …
Qays Ibn uhassir turned towards Umm Qirfah, who was very old. He killed her mercilessly. He tied her legs with a rope and then tied her between two camels. Then he made them run and they killed her. … 
Umm Qirfah was the chief leader of Tribe of Badr of Fazara. According to two reports above, Zaid Ibn Haritha went out on a journey to Syria and with them was Merchandise for the Companions of the Prophet (p). While they were near Wadi al-Qura, they were ambushed by men of the Tribe of Banu Fazara whose leader was Umm Qirfa herself.
The reports tell us that they were left for dead, which is what they thought. The things they were carrying (the merchandise) was all stolen from them. Another thing it tells us from the report is the only person to survive in this ambush was Zayd Ibn Harithah. The rest were killed.
Zaid Ibn Harithah with all the wounds inflicted on him returned to Madinah to tell the Prophet (p) what had happened. After a while (weeks or months, the report doesn’t tell exactly when) Zayd Ibn Harithah recovered from his wounds and asked for permission to take revenge on the Tribe of Badr of Fazara what they had did to him and his Companions. Permission was granted.
As expected Zayd Ibn harithah raided the people of Fazara and killed those who were involved in killing his Companions and robbing the merchandise. Umm Qirfa, according to the above reports was also killed for being the leader.
Besides the above, Safi-Ur-Rahman Al-Mubarakpuri tells us that Umm Qirfa wanted to kill the Prophet (P):
An expedition led by Abu Bakr As-Siddiq or Zaid bin Haritha was despatched to Wadi Al-Qura in Ramadan 6 Hijri after Fazara sept had made an attempt at the Prophet’s life. Following the Morning Prayer, the detachment was given orders to raid the enemy. Some of them were killed and others captured.
Amongst the captives, were Umm Qirfa and her beautiful daughter, who was sent to Makkah as a ransom for the release of some Muslim prisoners there. Umm Qira’s attempt at the Prophet’s life recoiled on her, and the thrity horsemen she had gathered and sustained to implement her evil scheme were all killed. 
Conclusion: It was not my intention to dwell into details whether these reports mentioned by critics was historically true or not. This can be dealt with maybe at a later date. Rather, my intention was to show the true circumstances as to why Umm Qirfa was killed.
The evidences shown, however barbaric it may seem, the woman was guilty of many heinous crimes. Not only did she lead other men to rob the companions of the Prophet but she also killed many of the Prophet’s companions when Zayd Ibn Harithah was ambushed.
The cruel method in which she was killed had nothing to do with the Prophet (p). The Prophet (p) never sanctioned such. Moreover, under any western law, the crimes she committed warranted death penalty or life imprisonment. The age of her would not make no difference.
Last but least, the claims made by missionaries and others that she was killed for no reason other than being a noble woman, and having followed her own old traditional beliefs, when we put these assertions to the test, we see the claims hold no weight. Umm Qirfa was a leader who robbed and murdered others. Hence, her fate came to an end when she was captured.
 The Life of Muhammad: Al-Waqidi’s Kitab Al-Maghazi [by Rizwi Faizer], page 277
 Kitab Al-tabaqat Al-Kabir, volume 2 – Page 111
 Ar-Raheeq Al-Makhtum: The Sealed Nectar – Biography Of The Noble Prophet [Revised Edition January 2002] By Safi-Ur-Rahman Al-Mubarakpuri, page 337