The fall of the USSR is very complicated and cannot be blamed on one one man or cause, but I'd have to say the main culprit is nationalism, not Gorbachev. In my humble opinion, a similar outcome would have occurred had Gorbachev never taken power.

The USSR's economy had been faltering compared to the west's for at least a decade prior to its fall. Their refusal to transition to a service-based economy, military spending that was far higher than it should have been, low oil prices, the failed August Coup by nationalist factions, and the failed war in Afghanistan were all contributors to this.

Meanwhile, all of the other Republics had been chafing at the dominance of the Russian Soviet Republic. They dominated politics and decided policy, often in their own favor, in other republics. So, there was an underriding current of resent against Russia within the USSR.

Gorbachev and other leaders knew their economy had to rapidly improve. They were looking at a scenario where the economy might crash and NATO would pull ahead greatly in the economic race. They enacted the big glasnost reforms that everyone knows about. Why he did this? Many reasons. Gorbachev was simply less authoritarian than the USSR was used to as a person. But the biggest reason was that the USSR was not doing well economically - and he knew that big reform was needed to do that.

One of the reforms allowed the other republics to increase their autonomy. Within some of those republics, social unrest began to occur and Gorbachev forbade the Soviet military to intervene, taking a clearly non-interventionist approach in management of the other republics. Some republics in the Baltics declared their economic independence from the USSR. Eventually, they were allowed to vote for their own independence and did so one after the other, dealing a fatal blow to the faltering empire. Shortly after this, the USSR was dissolved.

As for how he "got away" with it - he did so by getting power and wielding it. Like other Soviet leaders, he removed (not shot) from power those who opposed him and formed a government that was friendly to him. A lot of westerners think that the USSR was under a constant "great purge", where any politician who dissented was taken out back and shot. After Destalinization, this wasn't truly the case. A lot of that is western propaganda. After Stalin's iron-fisted control, many different political opinions were allowed within the party.

The important thing to keep in mind about Gorbachev is what he was not trying to do: he was not trying to cause the USSR to dissolve, not trying to institute multiparty democracy as its understood in the West, and not trying to replace the Soviet socialist economy with a free market one.

What he was trying to do was to jumpstart socialism. The OP talking about "executing people on the spot" does hit on a popular misconception about the USSR, that it was very much as it had been during Stalin's Purges in the 1930s. It was not. After Stalin's death in 1953, and a tumultuous decade of Nikita Khrushchev jockeying for power with other members of the Politburo and starting major campaigns that had disastrous or near-disastrous consequences (the Virgin Lands Campaign, attempts to reform the Communist Party, confrontational foreign policies), a more stable power structure was put in control with the ouster of Khrushchev and the installment of Leonid Brezhnev as General Secretary in 1964.

This period would see a so-called "Stability of Cadres" (or technically the Brezhnev term was "Trust in Cadres"), which basically meant that as long as senior officials in the party and government didn't rock the boat and literally toed the party line, they kept their senior positions. This in effect meant that the senior leadership of the USSR was frozen in place for a good twenty years from 1964 on, and that most of these senior leaders were of the "Class of '39", or were originally young functionaries promoted by Stalin at the end of the 1930s Purges (which is very much one reason why they wanted stability).

The senior leadership held on to its positions. Andrei Gromyko was Foreign minister from 1957 until 1985 (when Gorbachev "promoted" him out of the ministry and to eventual retirement). The Second Secretary was Mikhail Suslov, serving from 1957 to his death in 1982. The Defense Minister from 1974 to 1983, Dmitri Ustinov, was in various ministerial positions all the way back to 1941. Yuri Andropov was Chairman of the KGB from 1967 until 1982 (when he became General Secretary), and Alexei Kosygin was Premier from 1964 to 1980. By Brezhnev's last year, the average age of Politburo members was around 70, and these were not healthy leaders: Brezhnev suffered a stroke in 1974 and 1976 (when he was briefly clinically dead), Kosygin died in 1980, Brezhnev finally in 1982 along with Suslov, Andropov in 1983, Ustinov in 1984, and Konstantin Chernenko in 1985. In Brezhnev's final years he could work barely two hours a day and hold Politburo meetings lasting 20 minutes.

By the early 1980s, this lack of leadership at the top of the USSR began to be felt (as Gorbachev would later name it) as the "Era of Stagnation". Official corruption became widespread, alcohol abuse worsened across the Soviet population, and economic growth rates declined (with a recession arguably occurring in 1980). This was all while the economic and technological gap with the West was growing, the USSR was spending large sums on its defense industry (which was maybe a sixth of the total economy, if not more), and financing increasing adventures in Asia and Africa (invading Afghanistan, and supporting the MPLA in Angola's civil war, for example). The country increasingly seemed to need new leadership, and what Gorbachev would call "New Thinking".

Gorbachev and leaders of his generation came of age in the early 1960s, and were very much influenced by it, whether from Khrushchev's temporary "Thaw" in political discourse, or in the 1968 attempt to create "socialism with a human face" in Czechoslovakia under Dubcek (an advisor of Dubcek's was Zdenek Mlynar, who was a classmate of Gorbachev's in law school in Moscow). There was a general sense that party and government officials needed to be held accountable to Soviet citizens, and needed to jumpstart management of the economy to try to improve a standard of living that, while better than it had ever been by Soviet standards, was falling behind the West, and all at a time when full communism was supposed to have been achieved by 1980.