This answer is focused on the background to your question - the conditions which were in Russia at the time which meant execution of certain revolutionary groups were relatively uncommon.

Russia’s often-mentioned ‘backwardness’ did not apply to everything. (Case in point: Russia spent the majority of the 1700s ruled by women). Thus, contrary to popular belief, Russia has long had an on-off relationship with capital punishment.

Many Russian rulers, such as Ivan the Terrible, as their reputation suggests, were not squeamish over applying the death penalty, particularly to perceived opponents. For example, for Peter the Great, who ruled until 1725, the direct application of the death penalty extended to even his own son, Alexei Petrovich.

Yet as we move to the 1700s, capital punishment in Russia against dissidents was largely out of fashion, or at least, reserved for the most serious of cases, like the rebel Pugachev, who was decapitated and then drawn and quartered in 1775.

By the time the revolutionary movement began in the 1800s, this general reluctance to put people to death was still in place.

Humane alternative

Under Russian law, only the tsar himself had the power of life and death over his subjects. Noble landowners, meanwhile, could only themselves flog or detain their serfs, until the abolition of serfdom in the 1860s.

A more humane and standard punishment for errant members of the nobility, would exist right up the last days of tsarism, was exile, usually to the Urals or Siberia.

There, harsh natural conditions and deprivations may have felt like torture compared to the comfortable cosmopolitan lifestyles enjoyed by people of means and birth in Moscow or Saint Petersburg.

Back in the capital Saint Petersburg, this unwillingness by the tsars to put people to death for dissent was not matched by the growing revolutionary movement, which began to take further shape, paradoxically, after the abolition of serfdom.

To quickly summarise, in the 1870s-1880s, when post-serfdom attempts to motivate the peasants to rise up against their tsar and the system failed, some revolutionaries decided to turn to more violent methods: assassination of the tsar and his officials.

Bombing the tsar

In 1881, revolutionaries from the People’s Will, an early revolutionary faction succeeded in killing Alexander II, the very tsar who liberated the serfs, by a bomb blast.

Then, in 1887, university students in the People’s Will botched a plot to assassinate again by bombs Alexander III, father of the last tsar, Nicholas II.

In response, Alexander III’s government - which was far more reactionary since the death of Alexander II six years earlier - hung several of the plotters.

One plotter was Alexander Ulyanov, a young nobleman who was the older brother of a then teenage boy who would grow up to become Vladimir Lenin.

This incident had two main effects of deep consequence: by radicalising his younger teenage brother Vladimir Ulyanov, who before then had showed little interest in politics.

The now-radicalised Lenin, had learned, alongside several other revolutionaries, the futility of assassinations for political purposes.

Diverse dissidence

I write about Lenin’s brother to demonstrate a point: in the 1800s, revolutionary ideas, most among the educated, often noble classes, were constantly evolving and in flux.

This meant that not all revolutionaries were necessarily regicidal - and it was thus the more bloodthirsty dissidents who caught more heat from the authorities.

Consider what Lenin and later Stalin’s early revolutionary activities with what would become the Bolshevik party looked like - here’s a sample:

- Joining reading groups (essentially, ideological book clubs), and studying the works of seemingly nebulous figures like Marx, whose books were seen as little more than textbooks by the Russian censor and thus not banned.

- Creating propaganda pamphlets and discussion groups for industrial workers, who were themselves a small - yet growing - fraction of Russia’s population

- Writing polemics and arguments of the finer points of Marxist doctrine, which may have seemed to outsiders like arguing about the proverbial angels on the head of a pin

-Producing a dissident newspaper which presented events from a Marxist perspective

No hope

Seen from this view, the thought of the tsarist authorities reigning death from above on a fringe group of pedantic, divided ideologues might have seemed excessive.

Harvard scholar Adam B Ulam writes, in his book Russia’s Failed Revolutions, writes:

“There were people of high standing in the (tsarist) regime who tended to look through their fingers at the subversive activities of their young subordinates and acquaintances, viewing them as the results of misguided idealism and youth’s natural ebullience.

“Such a small group,” an indulgent police official was to say in 1895, after the arrest of a number of St Petersburg Marxists, Lenin among them. “Something might come out of it in fifty years.” Try 22 years instead!

A further point that may explain the leniency of authorities was the generally underfunded and under-resourced secret police, the Okhrana, whose job was to monitor and crackdown dissident movements.

The Okhrana was generally viewed with distaste by other tsarist government circles, including the tsar himself - a sort of necessary yet unpleasant solution. To use a modern phrase, there wasn't a lot of organisational buy-in within the government.

Furthermore, the late 1890s and 1900s were a period of rapid change in the world - and tsarist Russia, like its Ottoman and Imperial Chinese counterparts, was not immune to the liberal spirit of the age.

Old-style gallows oppression was no longer a go-to option for Russia , which was opening up to the West and its foreign capital, and thus wished to project a more modern, liberal image.

For example, the international reputation and moral authority of aging author Leo Tolstoy (himself a nobleman), who was clearly anti-regime, made him (and by extension, proponents of other viewpoints) less easy to sanction.

Last days of the regime

Now, you might say in answer to your question: Stalin wasn’t a noble - so why wasn’t he executed?

Stalin was a member of, nominally, the Russian Social Democratic Labour Party, which had since the early 1900s split into sub groups, the Bolsheviks and Mensheviks.

However, the 1905 revolution and compromising liberal reforms that Tsar Nicholas II had been forced to put in place legalised certain forms of political expression, such as political parties and groupings, newspapers, and so on.

This liberalisation, alongside the fast economic growth and industrialisation of Russia, and a sense of life generally getting better, was not particularly beneficial for revolutionary parties. After all, who wants a revolution in the good times?

Ultimately, the tsarist regime’s weakness was fully exposed in the First World War. When strikes in February 1917 toppled it, the post-revolutionary environment of both liberation and ongoing hardships of war created a liberal, but even weaker government.

This situation allowed Bolshevik propaganda and activities to flourish openly - and eventually, take full power for themselves.

Once in power, this relative leniency of the tsarist regime towards the death penalty - and the relevant lack of attention due to the secret police - would of course be corrected by the Bolsheviks, most quantitatively by Stalin himself.