Okay there is so much to unpack here. Do security agencies employ espionage and even outright assasination? Hell yes.
Do they however consider the assasination of one of the most powerful leader's of a nuclear armed nation? No. Not just no. Heck no.
Firstly, Putin is one of the most well protected leader's in the world. If he was killed it would likley be from an internal coup rather than a foreign force. He is known for his paranoia and his distrust for even his own fellow govt. ministers and supposed allies. Assasinating him would be difficult.
In the intelligence community the idea of a devil you know being better than the devil you don't is also another huge issue. If Putin was disposed of then this would create a power vaccum as the line of succession is murky. For all of Putin's terrible crimes he has kept Russia stable. Think back to when the USSR fell. The number of drug and weapon dealers which were spawned and used state resources was a huge issue and the effects are still plain to see internationally today. Just the thought of all the rogue elements given free rein if Putin disapeared is a nightmare for intelligence agenices.
Then is the issue of power struggles. An internall squabbling Russia sounds like a great idea in theory but would have huge international consequences (uncertainty over economic issues, food production, manufacturing, military, treaties,internal population management) and would make the likelyhood of a more extreme leader rising to power by a show of strength more likely. That would be bad...so bad.
All the intelligence on what motivates Putin and what works against him would be useless. The intelligence on his allies would suddenly become potentially worthless as intelligence communites scamble to figure out the new leader, their personality, quirks, goals and who their allies are. Also, the assumption taht Rssia would just quietly leave Ukraine if Putin died is naive. Putin has a number of supporters and backers who may take over if he died and assasinating him may allow a similarly like minded but potentially even more ruthless individual to take over. No government is just one man, no matter what it looks like from outside.
Anyway, all those issues are a headache if an assasination or a coup worked. Then you consider the disaster of if it failed. And that is a real possibility. Assasinations of high valued leaders in history have been seen to fail in the past e.g Castro. Heck, even when they succeed they can have horrible consequences like with Lumumbra sinne despite succeeding everyone knew who to point the finger at. If intelligence agencies try to assasinate Putin but fail then this would cause massive issues. Huge geo political and economic repercussions at the least, an excuse for more miliray actions and possibly even all out war in Europe.
Heck, even if it succeeded, if the intelligence agency was even suspected (as unless it was covered up perfectly Nato countries would be...een if it was covered up perfectly as a death by natural causes conspiracy theories and propoganda would likely thrive) of it this could turn Putin into a martyr and harden Russian attitudes. Cause a more hard line leader to rise, while giving an excuse for more retalitatory war and geo-political or economic repercussions.
Just...no. The intern or the new guy in the meeting suggests wacking Putin and then everyone else tells him to sod off and go get their coffee order while they have a real discussion. This isn't a Bond film.