In the 2nd season of Agents of SHIELD, why has Lash been killing all the other Inhumans, if he also had the ability to heal and protect them (as he did with Daisy 'Skye'), making them impenetrable to the parasite infection?
Initially, it was just something innate...when he killed he felt better.
Garner's body went through Terrigenesis, where his body became cocooned and every cell in his body was changed. As his body became freed, Garner felt as though he had completely changed and had an incredible desire to track down and be near any other Inhumans, although he did not understand why.
"I didn't know why, it was intense. I had this new desire, I had a hunger on my skin. It wasn't until I finally met one that my body took over and I, I lashed out and I felt relief."
Andrew Garner - AOS, Chaos Theory
Using Jiaying's book, Garner was able to track one of the Inhumans. Feeling an incredible desire to be near him, Garner approached and suddenly transformed into a monstrous grey-skinned beast. Garner's instincts took over and he killed the Inhuman by ripping a hole in his chest.
The Inhumans believe that every Inhuman was born for a reason, it seems likely (and I believe stated) that Lash was the counter-weight to Hive's influence...hence his ability to "cure" Daisy.
Part of this was as a result of Andrew Garner's ability to control himself when transformed into Lash which enabled him to use his Inhuman abilities without killing.
Note: The Marvel TV version of Lash is significantly different to the comic version where Lash embarked on a mission to find all the individuals affected and judge for himself whether they were worthy to live with their new abilities.
Wikipedia reference to Entertainment Weekly article in which...
Executive producer Jed Whedon explains that the show's adaptation of the character is different from the comics, but some elements are inspired from the source material.