Who admits to waging a war of aggression though? Putin is framing this as an act of defense, he says he is acting to prevent Russia's enemies from destroying it.
When in modern times has an aggressor ever framed the aggression in terms different from a righteous defense?
If Russia actually had separation of powers etc, it would be the courts finding that the war is indeed a war of aggression, of which there is of course ample evidence.
As things are, if he's still alive by the time regime in Russia changes, that will probably be (one of) the law he will be prosecuted under.
"Article 353. Planning, Preparing, Initiating, or Waging War of Aggression
1. Planning, preparing, or initiating a war of aggression - is punishable by deprivation of liberty for a term of seven to fifteen years.
2. Waging a war of aggression - is punishable by deprivation of liberty for a term of ten to twenty years."