To extend further on those tags on the last post, because I don't think I explained myself very well...
I see Gant as a man who has been subtly manipulating people around him for years without anyone really noticing. On the surface, he's an incredibly likeable person, but with an extremely commanding vibe to him. Your instinct around this man would be to be friendly to him and do what he wants, and it isn't hard because he does have that charisma.
I think that, especially when paired with Lana, he's hard to suspect. The Lana we meet in the game is the Lana who is being blackmailed into doing Gant's wishes, but before that point, she was supposed to be kind and nurturing. Even Edgeworth mentions that he felt like she took a special interest in him, and she wasn't even his boss until after SL-9 and Gant's blackmail. If Gant can be suspected while acting alone, surely the fact that Lana Skye trusts him will clear up any suspicions.
However, given that Neil's murder had absolutely no clear motive in canon, my first guess is that he began to suspect Gant's involvement in...something. Neil was smart enough to become a prosecutor, and he was a good one at that, since he won the King of Prosecutors award. It's possible he was clever enough to see through something and start investigating something that Gant wanted left alone. He didn't ask anyone for help because 1. he knew it was dangerous and didn't want anyone else in danger, and 2. because he wasn't sure anyone else would believe him.
When Gant walked into that room with three unconscious people on the floor, he had a choice to make. He chose to murder Neil. It's true that Neil's death would be the most effective way to manipulate Lana, but he could have just as easily murdered Darke and pinned that on Ema. Why would he choose to murder a friendly when there was someone he was desperate to imprison lying there too?
If Neil was poking around somewhere he wasn't supposed to and Gant found out about it, well, that would explain a lot. And I don't think it's really a stretch, either.
...but I also don't want to make Neil too smart and good and wonderful at everything, because then it's boring.














