@LyricalBallads98 I always try to make a point of not forgetting that some things are irrefutable "facts" while others are more "speculative" in nature.
You had a forced mate and your opponent's clock ran out: that is an irrefutable fact.
Your opponent "deciding to wait" though possible and perhaps even probable, is mere speculation on your part.
Any number of other explanations may apply.
If you want to give the guy the benefit of the doubt but can't be bothered, why not simply block him?
If you have corroborating evidence that shores up your feeling that he was sandbagging, perhaps you might usefully report him as well.
If there is one thing that life has taught me, it is that we are often a bit too quick to judge and, once having judged, disinclined to reconsider our initial judgment though it may prove to be misguided or overly harsh. This is not to say I condone juvenile or reprehensible behaviours or displays, quite the contrary. But being in the habit of keeping a level head and not being too quick to judge when more facts would contribute to an enlightened decision can't help but make the watering hole a nicer place to be.