There's no good answer, they thought the job was easier than it was and they'd be freeing up a few hundred billion dollars a year that they believed rightly belonged to their country. I'm sure in hindsight they'd do things differently.

The Saudis as you say did things the smart way, they started out with a 50/50 profit split with American companies and over time used their profits to take more control and increase taxes and eventually own the entire industry and diversify profits. That process did take 50 years though and Venezuela's government wanted fast results because they weren't an authoritarian regime like the Saudis and needed to win the next election.

Although Maduro fucked the economy so badly that he ended up turning Venezuela into a sham democracy and seizing authoritarian power because there was no chance of him winning a fair election. The road to hell is paved with good intentions.