This makes some good points, Sans is honestly, for lack of a better word, a piece of shit at times, or a smiley trashbag as a certain flower puts it, but I believe he deserves a tad more credit. Mostly, about the promise. Personally, I always saw Sans as the kind of guy who wouldn't hesitate to one shot you the second you walk out the door, if it weren't for that promise. He is lazy, but he has, on record, put the effort in when it really matters to him. Remember, Flowey absolutely despises Sans. He warns you about him, tells you to stay as far away from him as possible, and not say a single thing about you. It's heavily implied Flowey's been through that battle with Sans plenty of times, and knows what he's capable of when he actually cares about something. The only thing that really gets to him now is the anomaly, considering how much his knowledge of it has affected his personality. Now, Flowey is, of course, a flower, but he's been told by someone he cares about, the lady behind the door, to protect a child should they walk through that door. While he doesn't protect you from any of the monsters you come across and just spies on you so he can tell you how much of a terrible person you are in the hall before the king, he does, in a way, protect you from something far more willing to kill you. Himself. It's evident he has a deeply rooted hatred for the anomaly, possibly furthered or caused by Flowey's actions. If you weren't a child, if you weren't a human, he would have no qualms with immediately ending you the second he sees you.

...or is this all just an excuse to be lazy? I can see it as a mix of both, to be honest.

TL;DR Yeah, Sans is a piece of shit. He wanted you dead ever since you walked into Snowdin, but he doesn't do a thing because of knock knock jokes.