It isn't a scummy way to play, no.

A survivor who goes down towards the end of the match has fallen at a critical moment. Decisions become much clearer for the killer, because Borrowed Time, Decisive, body-blocking, etc. become much more powerful as soon as the gates and hatch offer the possibility of escape.

It feels bad for the survivor on the hook because the gens are done, and that seems like a win, but the endgame is part of the match. If you perform poorly in the endgame, you might not win, even if your early/middle match was fine.

The survivor's distress at a sudden reversal isn't the killer's problem to solve. Not relieving the survivor's distress doesn't make the killer scummy. There are plenty of cases when a killer has done well, but one moment in the game can reverse it entirely. That's not the survivors' problem to solve either, and not doing it doesn't make the survivors scummy.

The degree to which you feel bad about endgame camping is the degree to which you're unwilling to accept a 1K. Remember that survivors are a team. The survivor on the hook is holding the killer's full attention, and at the cost of just one sacrifice. It's not a big price to pay, even if you think the survivor should win.