So I'm watching Patrick slowly falling to Lord Gamblor's madness.
Why does he run out of the room when there's a fly instead of killing it? Haven't played much of BoI so not sure what's happening.
Haven't seen a lot of Patrick's videos, but flies that spawn from slot machines or the shell game despawn when you leave the room. If he's closer to the door than the fly it just might be faster, and depending on what items you have a single fly can be more of a an annoyance to kill than it is worth to deal with.
Also, if you spawn a fly in a boss room from a slot machine that you placed using a card, leaving the room without killing the fly and walking back in will cause the boss to respawn so you can get more items. I don't imagine that's what he's doing here though.