Imaam ibn Taymia had stated in ‘Majmu’ il Fataawa’ vol3, p. 283-284:

“If a Muslim is incorrect in his understanding about killing (a Muslim) or making Takfir (of a Muslim), then he does not become a disbeliever because of that.  This is like when Umar Ibn al Khattaab (ra) said about Haatib ibn Abi Balta’ (ra), “Oh RasulAllaah (sAllaahu alayhi wa salam)! Allow me to behead this hypocrite! RasulAllaah replied, “Indeed, he is one who witnessed (participated in) Badr. And you do not know if Allaah has looked towards the people of Badr and said to them “Do as you like, for you have been forgiven.” And this (hadeeth) in found in the two Saheeh collections. [ Bukhaari Maghazi 46 & Muslim Fadailu's-sahaba 161]

And in them is also found the narrations of “al-Ifk”: Usayd ibn Hudayr (ra) said about Sa’d ibn Ubaadah (ra): “Indeed, you are a hypocrite, arguing (on behalf of) the hypocrites.” So the two parties (Aws and Hazraj) began to argue, so RasulAllaah (sAllaahu alayhi wa salam) make peace between them.” [Bukhari Shahadat 15; Magazi 34 & Muslim Tawba 56]

These (Companions) were all participants in Badr and from them was the one who said, “You are a hypocrite.”, (and yet) RasulAllaah (sAllaahu alayhi wa salam) did not call him a disbeliever, nor the other, but rather he bore witness that all of them will be in paradise.

And similarly to this is what is authentically established in the two saheeh collections regarding Usaamah ibn Zayd (ra). He killed a man after he  (the person he killed) said “There is nothing worthy of worship except Allah”, and RasulAllaah (sAllaahu alayhi wa salam) severly reprimanded him when he heard about it and said to him, “Oh Usaamah! Did you kill him after he said there is nothing worthy of worship except Allaah? “ and he (sAllaahu alayhi wa salam) kept on repeating this question to Usaamah (ra) until Usaamah (ra) said, “I wished that I had not accepted Islam until that day” [Bukhaari diyat 2 & Muslim Tawba 159]

And along with this, he (sAllaahu alayhi wa salam), did not seek any retribution (from Usaamah) nor blood, nor expiation from him, because he was mistaken [in his belief about killing that man], thinking that that man only said those words to protect himself (from Usaamah’s sword). “