A lot of the Season 3 Episodes of the original Las Vegas were really disturbing. It seems that that was the most disturbing and gruesome season. There were a couple of other bad episodes in terms of disturbing storylines, but S3 was the most consistently disturbing season overall.
"High and Low", where the two neighbors were fighting and the one ran over the guy's dog in the yard was messed up in the extreme.
Someone already mentioned "Blood Lust" with both the mob scene, but the idea of the boy killing his mom's boyfriend, and the mom's boyfriend stabbing him, with the little brother watching makes me shudder.
"Forever," with the suicide pact and then the mother walking away while the daughter is begging for a second chance is up there.
"Random Acts of Violence," with the little girl getting killed is bad. Also for the effect on Warrick.
Lucky Strike, with the NBA player's 5 year old son, and the reasoning behind that crime was pretty awful.
"One Hit Wonder", and "Crash and Burn" are disturbing, especially because of the psychological effects on Sara.
"Feeling the Heat," from Season 4 was bad, just because of the baby angle.
However, overall, the most disturbing and gruesome episode for me, the one that I will NOT watch either on the DVD or in reruns is "Lying Down with Dogs." just the dogfighting angle, and the way they did the dogfight, especially with the dog that lost the fight....Let's just say they made it a bit too realistic.