Not so secret killing
Apparently they do. This is how the State of Israel planned to murder Khalid Mishal -
"Mishal’s murder had to be discreet and, if possible, invisible.The attack would take a matter of seconds – so quick he wouldn’t know it was happening.
One agent would shake a can of Coke and pop it open to distract Mishal while another would spray levofentanyl, a chemically modified painkiller, in his ear.
He would feel as if he’d been bitten by an insect; 48 hours later the drug would kill him, leaving no trace."
The Prime Minister of Israel agreed to this but the murder attempt was botched and subsequent events did not go well for Israel. Bill Clinton got involved, bottoms had to be kissed, they had to provide the antidote to their poison and Khalid survived and became a Hamas hero.
Of course, you will not have read about this in the British media.
The only other covert assassination attempt that I have heard of was the murder of Georgi Markov by the Bulgarian secret police. The British media found itself able to report that attempt because it was done by communists and the there are not many Bulgarians in the higher ranks of the British media.
When I read " 48 hours later the drug would kill him, leaving no trace" it makes me wonder how many other people have had an umbrella or ear spray moment.