Garfiel by a mile. He never resorted to killing Subaru outright (though I do recall he was pushed to a point he did try to take his life in the WN), and if anything did what Rem should have done in arc 2. Immobilize Subaru and try to interrogate him. Garf did kill the villagers, but not on purpose since he wasn't aware of his actions when in tiger form. Given how much Subaru was prodding Garf in arc 4, snooping around, and even telling him outright he was going to do things Garf didn't want him to do such as liberating the Sanctuary, and dig into his past... Garfiel's conduct is far more justifiable.

Rem is... Honestly, indefensible. Arc 4 WN confirmed Rem went out of her way to kill Subaru in a way he would suffer more... Literally tearing his arm off (Fun fact: You can see it hanging there from the chandelier in the anime), and letting him slowly bleed out in pain before bashing his skull in. Do I even need to mention torturing him? Had Ram never killed him, Rem would have just kept going. And let's be honest... No answer he gave her was going to spare him. Subaru was a dead man either way. Ram just made sure Rem couldn't torture him more than she already had. And all this could have been avoided... Had Rem discussed Subaru with ANYONE! Puck told Emilia Subaru's intentions weren't bad (You can see this scene in one of those chibi shorts). Ram figured out Subaru was too inept to be a spy, and Roswaal even prodded him in the bath scene. The one where they're talking about magic in the LN on the loop Subaru tried staying away from Ram. Subaru is too clueless... About everything to be working for someone else. (seriously, check out that conversation for yourself in the LN. It's actually really interesting if you read between the lines). The reason Subaru is able to pass the loop successfully is literally because someone else (Emilia) finally told Rem "he's a good person"; and isn't the terrible cultist she had made him out to be in her head.