I have a headcanon that Gallus’s backstory in the Guild has a breaking bad, better call Saul vibe to it. Like his life was less of a fantasy tale, more of a criminal rise and fall novel? Here are some details:
the only times the guild was really ‘thriving’ were back in 170s when Gallus ruled it. before and after him, they were a simple bunch of thieves, more or less. Gallus did his math, realised that (economically speaking) this ‘business model’ was unviable, and re-organized the guild into a vast syndicate providing all kinds of shady services
he won his reign in a competition against Mercer; the competition was to steal the Imperial Crown (right before the coronation of Titus Mede II). When Gallus returned to the guild, the thieves wanted to keep the crown as a symbol of their glory, but Gallus played a humble man and gave a passionate (and very much planned) speech about him not wanting to be anyone’s ‘king’ but being there to serve his people. Later, he sawed the crown into jewels, sold each one separately and therefore made an impressive starting fund for all his investments and renovations
He hired all sorts of young and impressionable misfits and trained them into becoming skilled copyists, gamblers, messengers, swindlers, and so on - so much that actual thieves became a minority of the guild members and its associates. Mercer sarcastically offered to change the name to ‘Chicanery Guild’ but eventually Gallus proved that shady land deals were worth A LOT more gold than stealing some gilded pots now and then
He had a young secretary that he personally pulled out of jail to work for him. The secretary was gay af and got absolutely infatuated with Gallus (who didn’t mind because additional loyalty never hurts)
Mercer and Maven started as business partners and continued as cynical lovers who enjoyed luxury and making fools out of people
Delvin and Glover, and later Brynjolf and Vex were a part of the ‘hire the orphans’ program. Vex joined the guild as a child who lost her family in the Great War, and was trained personally by Karliah, with whom she had an adorable big sister/little sister relationship. Brynjolf, in his turn, was Mercer’s young apprentice
Gallus had personal beef with the Empire because the Imperials jailed and executed Dralsi Indoril (his mentor and saviour, and Karliah’s mother) without a trial. So much that he intentionally financed Thalmor during the war (his biggest mistake that he came to realise when it was already too late)
Dealing with Thalmor required even more money, so Gallus resorted to drug trade (and later drug production, which became his very personal side business with Enthir)
He learned the Falmer language to keep his drug schemes and writings safe from everyone (especially Mercer)
Eventually he developed an addiction himself, but Mercer and Karliah discovered it and decided to take action. Mercer didn’t want to betray Gallus from the start: he stole the Key to save him
Using the Key, Mercer broke into Gallus’s safe fearing that he might have kept the stuff in there, but found his encoded trade plans instead. He deciphered them and got shocked that Gallus not only kept the drug money in the guild, but transferred it to the enemy of the state
Mercer tried to expose Gallus, but he and everyone else just laughed at his face. ‘What’s that, you say? Falmer writings? And we’re supposed to believe you’? THIS IS HOW ‘THE PURSUIT’ SHOULD HAVE LOOKED AT THE GAME
Trying to play bigger and bigger, searching for risk and fulfilment, Gallus dragged himself into a zugzwang. Thalmor had him, and so did the drug business, so when he discovered that Mercer stole and kept the Key (which meant that the Nightingales had failed, and Gallus was the one to blame), it was the straw that broke the camel’s back. ‘I am a dead man one way or another’
Mercer also didn’t plan on killing Gallus, he kinda saw it as an option. They had a super-complicated friend/rival relationship, and in fact, Mercer automatically rushed to save Gallus whenever there was a danger,
So yeah, they fought at the Snow Veil, but after defeating Gallus, Mercer felt something like ‘damn I can’t let him go’. And he would have saved him, but Karliah interfered, and the time was lost - and so was the chance
Mercer improvised and framed Karliah for murder, but he never really tried to track and take her down (that’s why she freely wandered around for 25 years)
In terms of behaviour, Gallus was somewhat of a Gus Fring type, and Mercer was a Lalo Salamanca one
Tldr: Gallus was super intelligent and curly and adorable but ALSO a slightly psychopathic risk addict who basically brought death upon himself, and Mercer was a cool guy who didn’t resist the temptation to become even cooler, and Karliah was a sweet cinnamon roll.