If Hitler had died before WW2, the world would have been better off
Is the same true of Putin today?
Imagine if Adolf Hitler has been assassinated in late 1938. At that time, Hitler had already annexed Austria and the Sudetenland, but had not yet invaded Bohemia and Moravia. Nor had he invaded Poland, the event that would start World War 2.
The immediate result is there would have been a succession struggle within Germany. Possibly Goering would win. But whoever won, it's unlikely they would start WW2, because the odds looked stacked against Germany and only someone incredibly reckless would have done so.
The 2020s look, to me at least, more and more like the 1930s.
In the 1930s the "content" powers -- USA, Britain, France -- didn't want war. But the discontented powers, who weren't happy with the existing geopolitical situation, did want a war if it would change it. Germany wanted territory in Russia, Japan wanted the mineral wealth of Siberia and/or the Dutch East Indies, and Italy wanted to control the exits to the Mediterranean, Suez and Gibraltar.
In the 2020s the "content" powers are again the USA, Britain and France, plus the other EU and NATO countries, and the rich countries of Eastern Asia -- Japan, South Korea and Taiwan. The discontent countries are China and Russia, both of whom see the USA and its network of allies as an enemy. China wants to conquer Taiwan which it sees as an illegitimate breakaway province. And Russia wants to conquer Ukraine, which it also sees as an illegitimate breakaway province. Should they ever succeed in conquering these "breakaway provinces" it is unlikely their aggression will stop there, any more than Hitler's aggression stopped with the Sudetenland.
I recently wrote that it would have made sense for the UK to get rid of Putin, in response to the Skripal poisoning (and Putin's many other acts of aggression against Britain):
An intriguing possibility would be if Putin were killed. He makes about 20 foreign trips a year and it might be possible to assassinate him, for example using a sniper rifle, drone, or anti-tank weapon.
I strongly suspect that over the coming years, more and more people will think the world would be a better place with Putin not in it. As with the scenario of Hitler dying, the immediate response to Putin's death would be a succession struggle in Russia. Maybe the new leader would be as aggressive as Putin, but i think they would probably be less aggressive, as Putin is playing a high risk game and most leaders would be more cautious.
Summary
I don’t know whether Putin will start a nuclear war, and I don’t know that another Russian leader wouldn’t. But Putin has shown himself to be a risk taker, and starting wars that seriously piss off nuclear-armed opponents (especially ones that’re more powerful than Russia, such as America) certainly counts as a massive risk. Putin is clearly an outlier in being willing to take risks, so I conclude that an alternative Russian leader would be less likely to do so.