I know that Lenin did have deteriorating health, but Stalin was a power freak who certainly had reason to kill him. So is there any legitimacy to that claim?
I've never heard this speculation (not saying you're wrong, I'm far from an expert on the subject) where have you seen/heard it?
I haven't really heard that one before. I've heard it claimed that Stalin was trying to keep Lenin away from the centre of power when he got sick, but not that he killed him.
IIRC Robert Service's biography of Stalin actually argues a reversed situation - where Lenin asked Stalin to assist him to commit suicide, but Stalin refused. From memory scenario this was based on correspondence between various individuals.