He's a nihilist. Thus he doesn't really have "motive" other than destruction and chaos. He believes fully in the concept that death and mutilation will reset the world... He's a parallel to Zero, what with Zero "resetting the world back to zero" and all that. He did release the english strain, to Quiet... who was originally working for Skull Face.
SLP was a project started to be just another weapon for Skull Face, I don't think it was really meant to do much... It was just another weapon like the Walker Gears, which he planned to use to exacerbate conflict, outside of Zero, and The Patriots AI plans. His goal in weapons development seemed to be a shallow cover plan. The Parasite plan was kept secret, and the reason he experimented with the other strains was because he was making sure it worked? I believe he had only recently found out about them, and was trying to study them... say... what language did he speak in nearly every scene? English. It would REALLY suck if he somehow got the english strain himself... He may have also been tailoring an english strain that was non-existent until he bred the parasites to adapt to it which might have been why he only had 3 vials.
It was dumb of him to use Tretij... but he seemed to think that his need to feed on hatred could only be satiated by his nihilistic hateful view of the world. Skull Face was dumb in a lot of aspects, having been duped by the two men he aimed to dupe himself... ironically he only ever faced Venom Snake, and his only real attempt at murdering Big Boss, was at Cyprus, and MSF... and like Skull Face, Venom is also quite unfortunate in that even knowing that Solid Snake a completely green soldier, who should have in all likelihood not have beat him... did. Both Venom and Skull Face are really dumb, and Volgin, Coldman, Liquid were all quite stupid... the only villains in Metal Gear with an ounce of intelligence are Big Boss, Ocelot, and Zero... the rest are power hungry tyrants, at least Skull Face was interesting in that he was the memetic clone of Zero, helping solidify Zero's shaky reputation in the future, similar to Venom do the same for Big Boss.