Good grief, reading some of these replies it's not hard to see why we're increasingly raising man children who need constant reassurance, round the clock entertainment and think the world revolves around them until well into their 40s. A nearly 5yo should be more than capable of being left alone for a short time without constant supervision, unless there are additional needs of course. Are parents never supposed to go to the toilet? Answer a door? God forbid, get their younger child dressed?
As for op saying she couldn't bear to look at him, that's literally why the term "get out of my sight" was invented. Show me a parent who has never felt this way and I'll show you a liar. The op's only offence was in admitting to it!
Op, I'd have felt the same. I have felt the same. My dd especially has pushed me to the brink (she's ND so life with her has been far from straightforward). But she knows I'd die for her and give her my last breath. She's also growing into a wonderful, kind, confident, independent and humourous young lady - despite me occasionally in the past having had to walk away from some of her worst behaviour in order to give myself time to calm down. And yes, shock horror, she was sometimes alone for 5 or 10 minutes downstairs while I had a poo or took a shower.
Op, the incident has passed and I hope you're feeling better about things. I do think it's quite unusual for a nearly 5yo to do what he did, but providing it's not a pattern of similar behaviour then I'd put it down to experience. I'd have maybe got him to help you re-wrap the presents, maybe taught him something about empathy by getting him to write you/ his sister a sorry card or whatever. And yes, I'd have told him that I was angry and disappointed - because those are valid emotions we all have!
If this is a pattern of behaviour you're concerned about then that's another matter, but my assumption wouldn't be that you're a bad parent or doing anything inherently wrong or neglectful. I've been there, after all - even with my eldest, much 'easier' dc!
In a few months/ years you'll look back on this and be able to laugh. Just like the time my dsis threw tea leaves all over the downstairs toilet (I mean the room itself, not just the toilet) in a fit of excitement about Christmas. She's a perfectly normal (well, mostly), fully functioning 53 year old now.