What a lot of people do in deathmatch is completely and utterly useless to them:
every time they see an enemy they default to panic crouching and spraying in the body.

And since they get some kills like this, they think it's the way to go - which makes them way worse than they should be.

HSDM would benefit lower elo players:

Adding HSDM would improve people's mechanical skills, both movement and aim, since they would get punished for crouching + spraying way more often. It would also improve crosshair placement of lower elo players, because if they go into a deathmatch and look chest / leg level, they will not get a single kill and they would realize they need to change something up.

HSDM would benefit higher elo players:

Other side of the spectrum is higher elo players. When they queue for a deathmatch, they're not there to shoot sitting ducks who do nothing but crouch in front of them. They do not improve from that at all - it's just a waste of time for both the high elo and the low elo player.

Preferably HSDM should come with all the other things people are asking for when talking about deathmatch:
-Way faster / instant respawn
-Get heal + full ammo when you get a kill
-Possibly more people in a lobby (i personally don't think that's necessary, i've just seen people ask for it)

There should also be an option to choose between Rifle and Pistol HSDM: https://imgur.com/a/551nroF - Maybe something like this could work.
(made in paint in 5 seconds but you get the idea)

I don't see a reason for there not to be a HSDM gamemode in Valorant.