From what I can get of the film, I don't know, to be honest. A lot of Krall's talk seems to be a case of why-I'm-bummed-Starfleet-isn't-military-anymore. I guess they were going for Krall/Balthazar feeling first misplaced by the Federation -- a bit of "what's my mission here?" -- and then abandoned, with a healthy dose of going mad from being stranded on a planet where his transformation began, but they seemed to sort of miss a few steps, a few beats. I think the idea of a disillusioned Starfleet officer really could provide some great opportunities to do a bit of meta on Starfleet itself and what it's about, the good and the bad, but it's kind of like we didn't get much of an explanation for things with Krall. For example, why does he think that using the superweapon on Yorktown is a form of salvation? (He mentions something about the weapon being the crew's salvation when he's holding them captive) I can imagine if he was also feeling like the Federation was ungrateful for all he did for them, but the film didn't go into that either. So I'm not sure what Krall's motivation was, honestly.

Hope I make sense by the way, a little tired.