if one is "unaccountable for" something it means that the person does not have to justify their actions, they are free from accountability or responsibility and need offer no explanation.
'Ostrovsky charges the Mossad with being dangerously unaccountable for its activities.''
Meaning the Mossad are seemingly able to do whatever they like with no repercussions . They are not answerable to anyone. Using the word dangerously infers some criticism toward the Mossad, this is not a good state of affairs.
You asked ''However these men remained invisible and largely unaccountable for their behavior.''
Meaning the men are not readily visible in society, and because of this it is not easy to have them answer for their behavior or actions. Like a secret society or underground group, the nature of this means the people are hidden from mainstream society, therefore it is harder to enforce the idea of being answerable for actions they may do.
You asked if it was possible to say "The men are unaccountable for."
I believe you mean "the men are unaccounted for " and this means that the men are lost, or their whereabouts are unknown.
"Accountable for" means responsible or answerable for ones actions. For example " You are fully accountable for the damage you have caused and will have to pay for all the repairs."
Deborah ESL Tutor