I've seen some posts here already mention that capital punishment is more expensive than a life sentence, so there's that.
Personally, this kinda boils down to who you are considering it humane for. This might sound a bit obtuse so bear with me.
For society at large, it might seem that a life sentence is more humane. It gives off this air of civilized, educated reasoning, of being above the brutality and depravity of the killers and murderers that they prosecute and, while they might find their actions abhorrent, they would never go so far as to stoop to their level and take a life in exchange for another. I've heard a couple of arguments regarding capital punishment that like approaching it from this moralistic stance, that society has this responsibility to be moral and just even when dealing with its lowest cretins.
On a more concentrated, individual standpoint, though, you could argue that a death sentence is more moral to languishing away in a toxic environment where the depressing and volatile conditions chip away at your sanity. I've heard a couple of people say that they'd rather see mass murderers and pedophiles suffer for years in prison rather than simply being offed, since they would theoretically die either way and the latter just prolongs their suffering.
I'm honestly undecided, but if someone pressured me for a response I'd probably be against capital punishment purely because I'm very squeamish and the idea of killing others is rather unsettling to me. However, it's a really nuanced issue and honestly there's no "good" answer at the end of it all.