It's a little difficult to say what exactly the それを助けとる part refers to without a little more context. This それ is presumably referring back to something discussed in a previous sentence - it wouldn't usually be used to refer to the enemy gang itself, but more likely a specific goal that they are pursuing, or a more general situation. I'd guess it refers to the overall circumstances that are endangering the speaker's gang.
The first part of the sentence is contrasting two positions held by the mentioned 連中, namely "手を汚す程じゃない" and "このまま立ち消えるならそれに越したことはない". So they're not invested in this cause enough to "get their hands dirty" (presumably referring to eliminating the speaker's gang themselves), but they do think it would be the best outcome if the gang were to just disappear (so they're tacitly supporting the cause of getting rid of them).
So the それに that you're having trouble with is fairly straightforward - the それ is referring back to the hypothetical situation referred to in the previous clause (このまま立ち消える, the gang just disappearing), and the に is part of the standard expression XXに越したことはない, meaning XX is the best possible outcome.
The このまま is a bit more fuzzy. As a general expression, it refers to things staying or continuing the way they are, so here I'd say it refers to the gang disappearing as a continuation of the current situation. In other words, the current situation is that the gang are in danger of being eliminated, and the 連中 would be happy if that situation progresses to its natural conclusion (the gang being wiped out entirely). It's a bit tricky to fit the exact nuance into a natural translation, but something like "...if we just disappear like this" might be close.
