Does he know?
HE KNOWS
Johan did see Tenma in that forest. I think this little game shows that Johan is perfectly fine either way. He lets some people and some things alone when they could prove to be his undoing; while also elimanating any threat or people that even get close to the truth. He's fine with giving fate a fair chance to thwart his careful plans. It's really all a game to him, why he's so scary and alien. Normal people don't see life and others like that, which makes him ill-fitted to understand or deal with people like Johan.
And for now, Tenma knows what needs to be done to him but he struggles to stop seeing Johan as a regular person. Even when he knows better. Even as a child suffering from a headshot wound, he was dangerous. Even behind bars, Johan could drive to their deaths his inmates and his guards, rule his new little world in a matter of weeks.
There's still so much we don't know about Johan, so it's still a little early to consider him a lost cause, while he's also done too many unredeemable things. (My man Richard. I'll never forgive that! "Would you like a drink? Tee-hee")