Regula didn't fear Varis - the pair had a strong friendship that spanned decades - and he was more than willing to turn his back on the flawed technology present in Azys Lla so it wasn't as if he was desperately seeking something to take back with him at any cost.
The initial bad blood between Regula and the Warrior of Light was based entirely on misunderstandings. Which is...exactly how things played out with Ysayle and pretty much every other antagonist that we've come to ally with after learning more about them.
At the end of the day, it is firmly established that Regula was a man of honour. Not simply within the lore book but through what we saw in-game.