@rudezki @Cyrenius @hikufalafel
I disagree. If he was a newly transmigrated teenager, then you could easily make the argument. But he's a sage with complete control over his mind and body at this point. We saw him have perfectly normal human emotions up to this point. He lost desires in the Eastern ascetic sense, basically vices such as greed, lust, pride and etc., not the ability to feel human emotion. He's not written as a cold-blooded psychopath and neither does he appear as one here because his demeanor is explained by existing backstory. He's the only character in the manga for whom it would make perfect sense to stay calm during bloodshed. Also I'm pretty sure he isn't decapitating people because it's cool. He's using the guillotine approach and killing people in the quickest and most painless way possible.
Whether the writer has done a good job on making him appear or feel as someone with hundreds of years of training under his belt is a separate issue. Ideally we should have witnessed more of the training for it to register with the reader better. He does look and feel like your regular Japanese kid with the exception of being extremely calm.