Questions & Answers about Ubojica je pobjegao.
It is nominative singular, because it is the subject of the sentence.
- ubojica = the person doing the action
- je pobjegao = the action in the past
So this is the basic subject + verb structure.
Yes, to an English-speaking learner it often looks feminine, because many Croatian nouns ending in -a are feminine. But some nouns for people also end in -a and can refer to a man or a woman.
Here, the form pobjegao shows that the killer is understood as male.
Compare:
- Ubojica je pobjegao. = male
- Ubojica je pobjegla. = female
So you should not assume that every noun ending in -a is feminine in meaning.