Board Thread:General Discussion/@comment-27620479-20160914150718

Let's get right to the point.

There is a joke, that good friends help you bury a body... Great friends bring their own shovel and don't ask any questions...

As for Sans, he doesn't help you bury any bodies, but calls you out on each one. He also asks questions... and subjects you to formal judgement. Compared to other monsters, if he really wants to be your friend, he's not really good at it. The judgement is already pretty messed up, as it seems Sans assumes you are a real bastard. Some low LV remarks are pretty harsh compared to what you could possibly have done, while high LV ones are pretty tame compared to what you almost certainly have done.

Then there is his death threat at MTT Resort, which is especially disturbing if you have done nothing wrong. Again, like he assumed you were a real bastard at heart, and wanted to keep you on the safe side.

There is also a chance that his other invitation isn't real either. There are multiple signs that you didn't even get to touch it – the most obvious is probably that the even the narrator breaks their habit of commenting on every meal.

The main evidence, however, is that Sans has reasons not to trust you, no matter how innocent you are. He knows that there is someone out there, who is playing with the lives of others – and is not above killing. It is of course Flowey, but Sans identifies them as you. He seems to know about the power to reset, that it has been misused, and that currently you possess it. What he doesn't know is that before you came, it was in Flowey's hands, or in his case, vines.

When Sans first invites you, he asks if you know about a talking flower. He seems to assume that it is an echo flower, but Flowey is a much better suspect – if you know about him. Given that Sans was able to force Flowey to reset whenever he found out about him, it makes sense that Flowey would try to hide his existence from Sans – and with possibly unlimited attempts, he should have figured out how to do that. Then there is the true pacifist ending, where Alphys realizes that a talking yellow flower is bad news – but Sans doesn't.

So, there you are: Sans doesn't know about Flowey, so he assumes you to be responsible for his actions. He makes considerable effort (in the form of threats and entertainment) to prevent you killing out of curiosity or boredom, but probably doesn't try to be your friend, until you seem to have decided on saving everyone – just like he doesn't try to stop you as long as you don't seem to have decided on killing everyone. 