Almost everyone regarded the new neighbour ....... suspicion.
( I would say that "with" is the answer. However, My question is that why we can't use "as"? )
"As" would mean "as if the new neighbours are suspicion". That makes no sense. A person can't be suspicion. A person can be suspicious.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.