This is a pattern I have seen a lot. They get a witness, forget about him for a few days, witness dies.
For example I'm watching season 10. They use a guy who bought a tiger to get closer to the actual villain of the episode, an exotic animal traficker. Stabler goes in with the 'witness' and leaves him alone with the murderers as soon as they figure out who they are. All Stabler says is "Come back when they have dealt with you" implying he know he is going to get beat up at the very least but Stabler doesn't care.
Of course after some leads we found out the guy was eaten alive by a Hyena and Stabler doesn't care at all because Stabler sees all criminals as subhuman apparently.
This is not the only case, Olivia has a lot of blood on her hard because she couldn't stay with the victims or witnesses for a few days.