I feel like a lot of people gloss over Héctor's flaws a bit— largely, I suspect, because he is nowhere near the most flawed character in this masterpiece, and so the flaws he does have pale in comparison to Ernesto, or even Imelda— but he does have them, and they are pretty crucial to the plot.
Firstly, he can be pretty irresponsible.
I mean, the whole reason he's with Miguel is because his reaction to finding a lost living child was "well, better spend all night coming up with zany schemes to get this kid to Ernesto" not "well, better report this kid to the relevant authorities, who will then be able to contact Ernesto and solve this whole mess".
I mean, one could argue that since Héctor has a rather complicated relationship with the authorities at this point, he doesn't necessarily trust them to help him, but then he's perfectly willing to take Miguel in after the announcement at the Battle of the Bands. It seems more likely that it just never occured to him.
Another example is his attitude to other people's belongings.
He borrows things from people— including apparently their bones (though, honestly, that one is on Chicharrón. After the mini-fridge, the van, the napkins etc. he really should have noticed that there was a pattern going on here)— and doesn't give them back.
Most people seem to assume that these were all confiscated at the bridge, like the dress, and a lot of them probably were. But then look at how he loses the second dress.
He comes in right as Ernesto is trying to send Miguel home and then when Ernesto doesn't recognise him he pulls the costume off, revealing his normal clothes— and tossing the dress and all the accessories aside.
He doesn't even look to see where they land. Which is, I suppose, understandable— he's kind of going through a lot right now— but it shows that he's not exactly careful with the things he borrows.
Héctor's irresponsibility sort of leads into our second, bigger flaw: it is fairly easy to talk Héctor into doing something he's uncomfortable or unsure about. The most obvious example being leaving his family.
Now, we never get explicitly told that Héctor was talked into going on tour, but it's pretty heavily implied.
The lyrics to 'Remember Me'— "though I have to say goodbye", "though I have to travel far", "know that I'm with you the only way that I can be"— suggest that he didn't feel like he had a choice about going, and then he outright tells Ernesto that gaining fame and fortune was "your dream, not mine".
This certainly gives the impression that Héctor was maybe less on board with leaving home than Ernesto was, but got persuaded to do so anyway. His fatal flaw is essentially not standing up for himself and what he thinks is the right thing to do.
And what I find interesting about this is that Héctor does not change in this regard.
At the battle of the bands, when Héctor finds out that Miguel isn't a professional musician, he is clearly visibly uncomfortable with the idea of letting him perform.
Which is absolutely the right reaction, considering that this is a life and death competition (for him and Miguel) and he has no idea what Miguel's definition of musician is, apart from the fact that it apparently doesn't include performing in public.
For all he knows, the kid's only just into his second week of guitar lessons.
His first instinct, to go on instead (and can we all just acknowledge the sacrifice he was willing to make in going on stage under the name 'De la Cruzito'?) is actually the sensible and responsible thing to do, yet all Miguel has to do is to say that he wants to earn Ernesto's blessing and suddenly Héctor's fine with it. Even after learning that the kid has never even done a grito before.
In nearly a century, he has not managed to sort out this flaw in his character.
Which I find really sad, since I personally headcanon that the decision to go home should have been a huge character changing moment for Héctor.
This is the moment where he finally realises that he can't just let Ernesto push him around anymore, because he has a family now and he has to stand up for them as well as well as himself. Like, there is an alternate universe where Ernesto doesn't poison Héctor and this is the moment where he grows up and becomes a more responsible person and stops letting people walk all over him.
But instead, he dies and is immediately cut off from everybody he knows and thrust into an environment where he has no real responsibilities and is desperate enough to let people talk him into anything if it means he might stand a chance of getting over the bridge.
Ernesto not only killed Héctor physically, but he also killed the person Héctor could have been if he had the chance to go home.
