Killing of so many people in Misfits?

It's definitely not a plot hole, but you are definitely not missing anything; it is simply a modified version of the reset button.

In the season 2 premiere, Nathan addresses the body count, "Can we please stop killing our probation workers?" as a joke, and then nothing happens.

It's South Park syndrome: while main characters can change (Mr/s Garrison) and people can leave and enter the cast (Tweak, Pip, Chef), there could be a massive body count and police would never notice save for maybe that episode.