Are there any downsides to killing thieves?

Since I began working for the Thieves' Guild, I've noticed the unnamed thief NPCs that show up in towns every so often to cause a bit of mischief.

Unfortunately for them, they aren't very adept, and tend to die rather quickly to the town guards' arrows - I don't really mind this, as they often carry shiny things that aren't marked as stolen and no one minds me taking them.

If I were to hasten their demise by accidentally brutally stabbing them to death, would anyone mind? Will the Guild make any reprisals? Is it better to just wait for them to blunder into a guard?

There are no downsides that I've noticed.

My character is a Thieves Guild member (now the leader) and tries to kill any unnamed thief that randomly shows up in towns (mostly in Riften) and receives no repercussion whatsoever from guards, Thieves Guild members or any other NPC. The random unnamed thieves carry assorted valuable gems and other items, so you might want to loot their body when they die.

The only thing you got to worry about when attacking a thief is accidentally hitting, or worse, killing other NPCs, which will net you a bounty (nearby NPCs like to gang up on the helpless thieves, making it likely for you to accidentally hit them if you join in the attack). So don't hit that thief with a Fireball. =)

When you kill a thief in Skyrim depending on the guild, their group will set a bounty and you will be chased unless you stay off the Skyrim grid(hide).