Reiner feels guilty about murdering everyone despite being a titan? why would a titan feel guilty about it?
Monkey trouble is apparently their target , a halfway important honcho in the titan hierarchy, and he can turn humans into titans. Titans transformed by him are the more traditional types as well.
So back to the question of why team BRA would attack the walls?
If Monkey Trouble is converting people into titans then what would be the most efficient way to stop him?
Kill them all.
The entire country Eren lives in is a titan factory.
Mankind was only left alone for a century to increase their population and make more titans.
Sending out the people in that halfassed plan to retake the first wall was just providing monkey trouble some new subjects that were denied to him when everyone evacuated.
The second attack on the walls was because monkey trouble was ready to start converting again so repeat plan A.
The Royal family are the administrators of the factory and the top fighters go to the Royal guard because near the final stages people might figure out who's behind this and the admins want to keep themselves alive from the riots.
Everyone is a potential titan. Eren's dad simply had a way to control it.