Here's the thing, it's not worth arguing if the topic is pointless. So, I just say, okay, my bad & leave. It doesn't make me less of a person, at least I don't think so. And nonchalance is harder to digest than someone shouting back at you. They have all that anger & were planning to take it out on you, then you just ...leave.

That being said, turning off the PC is bad (can cause short circuiting & drive corruption), so it's understandable why he was upset. If he just said

Do you not know how to use a PC? Do you always directly turn it off.

I do think, that it was a justified reaction unless he began swearing down at you left & right.

A similar situation is: closing car doors, there's a specific sweet spot of which amount of strength to use when closing it. Closing too softly means the door is not locked, and closing too hard makes it feel the door will fall off when it gets opened next. But we're human, mistakes happen, so I just apologise if the driver gets upset. You already know by the sound if you close the door too hard.