All nations are interested in the ability to remotely control the pacemaker or the leaders of all other countries. We should assume that Russia, China, and the US are all able to pull it off at will if it is possible.

Would they every use this? It is an act of war, and they risk the replacement leader being "worse". However it is something they would like to have. Of course once someone starts using this all medical device makers will have to take security seriously, which enters an arms race.

The only protection most people have is we are not important enough to target. However if you are a world leader of some sort you are always at risk that some other country will decide the best way to deal with you is kill you. Vulnerabilities in your medical devices is just one possible choice such nations have.

Then again there are lots of other choices for murder. Poison has been used going back thousands of years has the useful property that you can make it look like an ex did it thus avoiding war.