"...And that there was something in it for me if I'd help 'em out. He said that - he said that you were bein' tough on the negotiations. But if they could get a little help and close the deal fast it'd be good for the family..."

These are Fredo's own words from the boathouse conversation. The first thing he mentions is the offer of something in it for him if he helped THEM out, because his brother the DON was being tough on negotiations. Stupidity was certainly involved, because if Fredo had an iota of intelligence he might have realized that Michael had his reasons for being 'tough' on negotiations. But while he never foresaw or inteded for Michael to be killed/injured...it's clear from his confession that Fredo put his own gain ahead of that of the family.

I don't think he was trying to prove to Michael he could handle something on his own...because if that were the case he might've thought to report to Michael that he'd been approached by Ola with that proposition in the first place. If he'd looked an inch beyond his jealousy and resentment at having been stepped over, he might have thought that Michael should decide what was 'good for the Family', not Hyman Roth & Johnny Ola.

What hooked him into working with Roth's people was the promise of 'something in it for him'. On his own (more of Fredo's own words).

We can see in the boathouse scene that Michael doesn't have to do much to draw out of Fredo the underlying reasons behind the betrayal. Ignorance and stupidity were of course factors in Fredo's decision to work with Roth. But he had been raised in this business. Others have mentioned that he might not have worked with Roth against older brother Sonny under the same circumstances.

Malice and intent and personal gain, over the Family, over his brother were the prominent and underlying reasons for what Fredo did. He admitted it himself. There's no way around it, even if he didn't realize it was going to be 'a hit'.

Apple