Yeah it just makes you an asshole. It's not illegal.

There was a pretty active Ask HN a couple weeks ago about a very low quality employee in line for a vesting cliff. I think 0 people fessed up to "let them drop." Almost everyone said the only moral thing to do & the only thing that wouldnt make the rest of the company hate & distrust you was to offer a pretty generous buy out ahead of time, at basically the current value.