Aaba is indirectly responsible for everything that happened to Tulsi at that deserted factory. His intention was to murder Aryan, he does not get a chance maybe because he gets startled by Tulsi's presence so the duo took advantage and shot Tulsi. But the point remains that the duo's entry occurred only because of Aaba. He chose to ensure Poorva remained behind so she can inflict more nonsense. So of course he is the culprit. Sometimes the biggest culprit is not the shooter but the person who ensured the circumstances began such to enable the shooter to shoot. Plus Aaba even physically assaulted Kumudini, tried to kill them plenty of times in the past but kept being unsuccessful, so lets not forget those instances. Kumudini is also responsible for Tulsi's death, had it not been her games, she would still be alive. So we can blame both parties here.
Aaru supporting him is understandable as she found out he did not shoot Tulsi. But I still find her behavior disgusting as Tulsi had so much faith on a loser like her, who has no time to mourn but rather to defend her own blood related family. (Yes she annoys me these days😆 Just like Aryan used to😆)