Akame ga Kill!, Ansatsu Kyoushitsu Recommendations
Shounen/action manga about groups of people whose goal is to kill a certain person and set everything straight. Akame ga Kill is much, much darker than Assassination Classroom, but their art styles both include some darker panels. Both have large character casts and well thought-out development.
~Ansatsu's more of a light-hearted take on the concept of assassination
~Both Mc's go through a really dramatic in character development