Resons I think Tommy Merlin IS Sara's killer.
1.Laurel is on A MISSION to kill whoever,whatever killed her sister.Tommy would present the greatest conflict to her tunnel vision.Think EMOTIONAL CONFLICT.
2.A weapon AGAINST THE MAGICIAN.Tommys death was Malcolms fault.A resurrected Tommy is a doomsday device against him courtesy Ras al ghoul.
3.Oliver.also reference #1.Oliver and Laurel screwed each other right in front of him,killing Sara pays that betrayal back in spades.Oliver,like Laurel & Malcom,would be TORTURED AT THE SITE OF TOMMY.
4.Ras al Ghoul makes all this possible.I know the showrunners said no lazerus pit,but it woul tip thier hand especially in if This is what they're doing.
5.What other scenario would impact ALL THE MAJOR PLAYERS IN SUCH A EMOTIONAL WAY?I haven't even got to the effect on Thea.
6.The pit is a one shot thing,given Arrows more grounded nature,if it can only be used ONCE,this is the most impactful way.