turn what they come for against 'em

[Jeff Jacobs]

Dick warns Danny about present world:

Dick: Swarm it like mosquitoes or leeches. Can't do nothing about that. What you can do...is turn what they come for against 'em.

What's the meaning of "turn what they come for against 'em"?
Source: Doctor Sleep 2019

 

[Uncle Jack]

I don't think we can say without having some context. Who are "they"? What do they come for? (Or why do they come?). Does "it" at the beginning refer to anything in particular?