Um, the assassination of a head of state by another state would be considered this yes.
Can you imagine how the USA would react to a radioactive isotope in Biden's tea?
When the clause was put in the analysis was what I said.
Not a threat to the existence. If Biden got killed, there is Kamala Harris. If she bites it then McCarthy, Murray and so on.
You know what would be a threat to the existence of the US, setting of a full scale nuclear war.
When Kennedy was shot, the US lived on. It wouldn't have made a difference if it was a lone gunman or someone working for the Grand Duchy of Fenwick.
Ditto for Poutine. If he was to die, someone else would take over. Life would go on. World leaders die all the time. No reason to murder 100 million people.
Now is a foreign power assassinating a national leader and extremely hostile act. Yeah buddy. Does it warrant a DoW, perhaps, if you and friends can take on the enemy. You kill Castro and Cuba ain't doing shit because it can't. If Cuba killed a US president, there is going to be some very serious payback.
However it would not qualify for a threat to the existence of the Russian state that would allow for a nuclear response FFS.