Stupidly to poison someone, and then give them to other country.... 0_0 Say someone is a leader this less then 2% stupidly too...
Putin isn't afraid of the foreign response. He's afraid of domestic enemies gaining enough confidence to attempt a coup. Domestic enemies are the sole threat to Putin. He wants them terrified; he wants them to understand that he will kill them as necessary. Putin knows very well the first rule of being a dictator (which Lukashenko has forgotten): if you want to be dictator you're going to have to kill people, and if you want to remain dictator, you're going to have to keep killing people so long as you want to retain that position.
If you're Putin, ideally Navalny lives and is grotesquely destroyed, broken, by the poison; an example for all that would challenge his power. It says: I can do this to anyone in Russia at any time, I don't need to be very subtle about it beyond minimum plausible deniability, and there is nothing the world can do to stop me.