Plan B of Crey? It wasn't that easy for him to resort to self-explosion, hence his earlier indecisiveness was justified. For starters, he didn't have that hard resolve to kill her, and second, he was already having cold feet, the minute he saw his plans turning useless.
And for the following main reason, the alarms ringing around the Academy were the explanation. Though, if circumstances weren't as bad as they are now, even if he had a choice to detonate her without hurting himself, he still would have a reasonable apprehension to use this backup Plan.
Because of the ruckus, this plan would have been created. If earlier, his operation was transpiring in complete secrecy then after such a large explosion, the whole Academy would now be notified about this. There was the reason he wanted to leave immediately after this incident but after such a loud commotion, forget his survival, he would never be able to escape the MILITARY Academy.
