What makes criticism constructive?

People claim that saying "you're bad" is constructive criticism, rather than being harassment.

Well, that's not true. It's criticism, but it's still harassment on the grounds that you're not basing it on anything and therefore it becomes an insult. Qualifying it with "You're bad at X role" still doesn't become constructive. So what do you need to do to make it constructive? Be more specific while keeping insults out of it.

"You CS worse than a one-handed retard with spastic hands" is harassment. It's clear where the player needs work, but you're still turning it into an insult.

"Why didn't you go in?" is borderline harassment. It aims to put full blame on the other person rather than attempting to see if the mistake was on the speaker's part. If the person going in didn't see why the situation was dangerous or why he didn't need to go in, saying this means they are not looking at the possibility they made the mistake. I've dealt with this a number of times on bloodthirsty supports and a few have trolled over the misplay they made because they didn't (and often refuse to) see why they were at fault for that.

"Stop fighting" is arguably harassment. It implies that a lane is continually choosing to fight outside of any safe zone rather than being tower dived constantly. If someone is dying repeatedly under their turret, more than likely they're avoiding fighting already but are losing lane badly enough they can't stop dives.

"Stop dying" is harassment. Unless a player is super high or inebriated, chances are they're fully aware that they've died and died often. Perhaps they can't catch a break. Perhaps they're being targeted by a fed assassin they don't have the tools to deal with. Maybe they're just making a ton of stupid plays because they're tilted and/or desperate to make a comeback. In some cases, they're using their low gold reward as an excuse to split push because it means the enemy will gain less from splitting off to kill them and they don't bother fighting back because they know they won't win.

Constructive means give them something specific to improve on and without insulting them. Lines like the above are toxic because they don't give a player something to focus on doing better and implies that said player is unaware of what's going on.