I believe the Traviss books say that he got command of Infinity because he was the guy in charge of its construction in the Sol oort cloud, and proved a pretty good bureaucrat and administrator. Unfortunately, while they're good skills some someone in a desk job, it doesn't qualify him for command of humanity's flagship. Ironically, ONI were the ones who wanted Lasky in place as XO, because he was the guy with the real talent, and it was the regular Navy who decided to keep Del Rio on, which I presume means he got the official nod from Lord Hood himself. Given the significant vilification they've been getting from the EU lately, I'd expect it to be the other way around.

The story goes to some lengths to paint Del Rio as incompetent (repeatedly assign his XO to field operations, the original recon mission, the complete ignorance of force recon while hitting the particle cannons, his treatment of Cortana's rampancy) but instead, every phase just makes me sympathise with the guy more. He's a desk jockey who never expected to be facing glowy-skull faced robots up close and a Peter Jackson knockoff Forerunner, who probably knows Lasky is there because ONI has no faith in him, and has his authority totally undermined when he orders Lasky and Palmer to take Cortana and arrest the Chief and they just stand there, gaping. Worse, Lasky commits what is tantamount to mutiny by giving the Chief a Pelican, and gets promoted to his job, while he gets a firm boot in the behind for trying to take what he saw as the sensible course of action. It's not hard to see the guy harbouring some resentment, and even finding some support among those who think Hood shows an improper degree of favouritism to the Old Guard. Do I think he was right to take those actions? No. Do I think he was justified in his behaviour? More than a little.