I can't quite figure out why I should feel bad for people who get themselves killed due to their violent criminal activities. It's about on the same level as the idiots who fairly regularly electrocute themselves while trying to steal copper. I don't particularly mourn their passing.
The innocents they may have poisoned in the aftermath get my sympathy, of course. But that just reduces my sympathy for the criminals all the more.
I don't think it is when decided and inflicted by a third party, but self-inflicted death on grounds of aggravated (c|st)upidity is hard to feel sad for.
It's hard to make judgement without all the facts. Perhaps they were blackmailed into doing it, perhaps they were evil, greedy, disadvantaged in life... who knows.
The innocents they may have poisoned in the aftermath get my sympathy, of course. But that just reduces my sympathy for the criminals all the more.