There's absolutely no need for violence to get everything they need. Here's the deal that could easily be brokered:
- We give them a place to land and set up a temporary base
- We give them (or help them get) the resources they need
- We get to observe at least some of their tech in action
- We get to see what kinds of resources they use
- We gain the knowledge that FTL is possible and can be practical
- We get a massive head start in developing similar technology, with the chance that we could reverse engineer some of their tech within a short period of time.
Basically, even the chance to observe their tech is worth giving them everything they want. The only thing they have to do is not use up their resources destroying us.
If you really want the captain to be a homicidal maniac, he really doesn't need to do much damage. All he needs to do is sufficiently intimidate us to leave them alone. Simply nuke a couple places near where he wants to set up a base, and leave his big ship in orbit. Then send out the message that if anyone tries to get close, they (and the city they came from) are gonna get nuked. We would not know what their tech and supplies are like — we don't know how much destruction they could do before running out of supplies, and we don't know how good their sensors are so we don't know how hard it would be to pull off anything (even sneaking into their base to just peek at their tech).
Your captain can make good use of bluffing, too. We would have no way (or desire) to check a claim that he had a weapon that would cause a grey goo scenario for us. He'd only have to sell it as something that would be rather inconvenient for him.