Why was Maximus relucant to help kill Commodus?

Two things to note on that. One is that he has lost most of his faith in both others and himself. At first he doesn't know if he can trust Lucilla (though by the end of the scene it seems like he feels he could), he doubts the abilities and resolve of people including the Senators given how Commodus has been shown to be able to get away with all he has done (It being established, though better so in a scene from the extended cut, that Commodus has declared martial law. Getting the loyalty of several Senators like Falco to his side, as well as having his Praetorians capture and throw into the arena many of those who openly speak out against him. Those against him like Gracchus, Gaius, and Lucilla shown meeting in secret.), and also notable though largely unspoken but still present is that he did actually start to like the adoration of the crowd as Proximo suggested the gladiators would earlier in the film when speaking to them before their first big fight in Zuccabar. (Such as after the "are you not entertained fight" when Maximus starts to walk out of the arena but stops in his tracks and to look up and soak it in for instance, or look at his expression when he looks up to the crowd after his big reveal in Rome) After beating Tigris of Gaul and after deliberation ultimately relenting in killing him, which helps restore faith in himself as being able to be the man he once was and also giving up giving into the demands of the crowd who urged him to kill as usual, he decides to take a chance on Lucilla's offer as well. Or at least that's how I see it.