Tom didn't so much disagree that Roth should be killed, more the probability of successfully carrying out an assassination on a very well guarded man.

I don't think we can know Tom feels this way about Fredo, when Mike calls Fredo weak and stupid, we don't see Tom nod in agreement, also, when Tom says Fredo doesn't know anything....had he been against Freddie, he'd have told Mike plainly that the guy has got to go

We just don't know if Tom LOVES Fredo - thats the key.