Yeah, even in cases where people didn't positively expect a person to go shootyhands, they're often not really surprised to find out they did. The killings of Allison Parker and Adam Ward during a live news broadcast, and the Virginia Tech shooting, are examples of perpetrators like this.
But, complete surprises do happen. As far as I know, to date, no indication has ever been found that would've suggested the man who committed the Austin package bombings in 2018 was prone to do such a thing. Police found a confession video the perpetrator made, where he describes how he made the bombs, but even in this video he reportedly never gave anything resembling a motive.