compost

I have discovered there are ants in my compost bin which surprised me
because I thought the temperature would get too high. I have a massive
compost heap at the bottom of the garden but this is just one of those
plastic bin things.
I've also been informed that someone put any killer powder in there a
few months ago. So, 2 questions I'd love answers to, why are there
ants, I've never had a compost before where I could see 'creepy
crawlies' in it & secondly, is the compost safe to put on my herb
garden, since it's had poison in it.

I've been trying to grow organic herbs but feel this ant-killer powder
has compromised that.

I'd be grateful fo rany help with this,
Chrissy