Well he or someone has to because without Vandal Savage LOT would not happen. Altough it was kind of weird that why would they need more heroes and villains to fight Savage in LOT if they were able to defeat him by team Flash and Team Arrow joining forces with the "Hawks"?
If I were in charge of these shows I would've made it that Vandal would only send one of his minions and let the minion fail. Vandal failing like this only gives bad rep to LOT before the show even begins. Well it's their folly.
On the other hand
With the Lazarus pit corrupted by Nyssa I totally can see that Merlin is freaking out and seking out every possibility to extend his miserable existence. He also fears that Nyssa may escape the dungeons and collects his head. Although that's not likely, not in this show.
This is a new low even for him. Even if Savage has not lived up to the expectation to be a worthy Spinoff villain on LOT (at least in my book), he is by far the most capable villain of either show. Although Zoom may have done easy work on him by his lone some (well minus the immortal part). Maybe Barry could ask Vandal to neutralize Zoom? More likely they'd team up and kill Barry instead.
Team Arrow and Team Flash will surely kick Merlin's ass if they find out what he's done AND I will kick theirs for leaving Vandals ashes behind. Not unlike the chemical bomb.
Leaving the security guards to be taken out by big bad of the season (Darkh) only to make a "big" entrance after is usual everyday Arrow BS, but to just leave the bomb like that, later act like nothing happened.
"Dude, you were ready to die to protect it a minute ago!" In fact if Barry haven't showed up Ollie would be history right now.
Then leaving behind the ashes of the immortal dude. Honestly I was waiting for Vandal to pull a Voldemort and going trough everyone's chest in a dust-ghost form, punishing them for their stupidity.
However I knew in my mind that after this f**kup the writers needed a plot device to bring Vandal back, somehow.