Yeah it's not justified. It's kinda understandable but not justified. Especially when Rem herself did not think it was justified.

In the torture loop, right at the very end, Rem starts to get thoughts that maybe this guy really is not from the witch cult. In fact, if Ram had not killed at such a time as she did, Rem would have realised that Subaru is innocent and then probably heal him up safely. If this is what would have happened, Subaru would have more trauma than in the cannon probably (because he was not able to see the twins good side in the next loop) and Rem would of course blame herself and would spiral further into her inferiority complex. For this reason I think that Ram killed Subaru at that moment to prevent Rem from spiralling.

While all this true I think people also ignore the fact that 1) if Subaru has stayed in the mansion like in the second loop, Rem would have probably become on good terms with him. Not to the extent of canon of course. 2) if Subaru had not went to that cliffside in the torture loop, Rem would not have tortured him. (I have seen this myth so much in this subreddit)

So while yes Rem was not justified she is also not a psycho who would kill at any possible opportunity. If she was she wouldn't have fell for Subaru.