- He was left in a bitter cold and so he is bitter at them.
- He was left outside in a bitter cold and so he is bitter at them.
Are both correct?
It should be “in the bitter cold”.
The repetition of “bitter” in two slightly different senses is very noticeable and, in my opinion, undesirable unless it is your deliberate intention to make a play of it.
Can I say? : “He is bitter to them.”
No. You could say “bitter towards them”.