A very 'thoughtful' question!
They are all sentences in which there could be choices - depending on your attitude towards the group of students, attitudes, reactions, personalities, or soldiers. There is no fixed rule that such sentences use a singular, or always use a plural.

In the first one (because you say that the attitudes are similar, not the same), you say that there are several attitudes - which calls for a plural

1) The attitudes of all the students regarding the election were similar.

However, in the second, you say that they all had the same reaction - there was only one reaction and they all shared it.

4) The reaction of all the students regarding my speech was the same.

In the last one, you are treating the soldiers as individuals, with different personalities, and so it really calls for a plural - "The behaviours of the soldiers in such a situation depend on their personalities."
However, that sounds rather strange.
"Behaviour" is normally uncountable. You could say "There were three soldiers, so we saw three behaviours", but it is definitely not common usage.
It would be more natural to phrase the sentence to address the soldiers individually - "The behaviour of each soldier in such a situation depends on his or her personality."