What do we do if SETI is successful?

As a pragmatic opportunist

- Setup a massive array of antennas in space for reception only

- Try to decode their radio traffic and understand how they are exchanging information

- Steal their their knowledge and use it to advance human race forward.

- Reduce all our electromagnetic emissions to minimum to deny them the same advantage. Forbid anyone from sending signal towards them so we have time to technologically catch up to them without them noticing.

Any kind of contact will ends up in abysmal disaster as we have seen in the past, when advanced civilization shown up on shores of less advanced one.

This presumes they have the same nasty survival-of-the-fittest kill-or-be-killed attitude as humanity. Our evolution kinda created that but it doesn't have to apply everywhere. I think it's entirely possible that alien civilisations could exist that are a lot more symbiotic.

We have a saying in Holland "the innkeeper trusts his guests like himself" which seems to apply here.

Right; or, since they are not competing with us for resources, they could kill us just for sport.

Again the concept of sport imposes human concepts on a hypothetical alien culture.

There's no reason to assume their society would have developed along similar lines. I'm sure there's alien civilisations that are more aggressive than us, but also ones that are less so.

I don't think we'll ever meet any though as our lifespan is just so short on a universal scale. And FTL travel seems to be impossible otherwise we'd have seen signs of it.

Of course according to our current physics understanding it is also impossible but I don't think humanity is very smart yet. But this thing might be right.