With the infiltration mission on its way, it was good to see each of the students putting their own skills to the test. As each of their teachers was handicapped or preoccupied. Koro-sensei calling it a summer assignment has merit because the students are applying the skills they learned.
We end up learning that Takaoka is the one who was behind it. Considering what we know about him from before that, his strategy is mostly as a brute. Thus the assassin that was hired was used at their full potential.
At the end of the volume, we see he wanted a petty revenge against Nagisa. Still in the final moments of the volume when Takaoka blows up the antidote, we see that triggers Nagisa to no surprise. So will he take the plunge into being an assassin, or will he be able to hold off from killing?