Odd, for a short period of time after his dad died, was very callused/spiteful in how he interacted with many people, so he said quite a few things to people he cares about he regrets

One of the worst things he said was to his friend Linus (a smarmy opportunist and most recent inheritor of his family's connection to Yveltal), whom had gotten in an argument with Odd over the things he was doing to try and find who was responsible for his father's death (something Odd felt particularly vindictive about in regards to Linus, as they hadn't proved themselves to be any paragon of morality in the years Odd had known him). The fight escalated, with Odd ending it with this;

"You know, maybe you weren't so wrong, what you said all that time ago; maybe you are better off dead."