Provocation cannot justify Sameer's act. Surely, there was provocation from Rishi, Indira, and Mandira at various circumstances as pointed out.
I think a person like Sameer who is abnormal either somebody provokes or not, his nature of behavior is going to be the same. Do you think if Indu had not slapped him, not taunted him in front of his employees, not called him gutter ka keeda, he wouldn't have had any obsession to acquire her ?
He hates losing and I suppose Rishi knew Sameer better than any of them and it is sometimes questionable knowing his psycho behavior of his brother, I wonder why did he threw that challenge who wins over Indira ? or was it Rishi taught like his many ex-girlfriends which Rishi took over from Sameer and Sameer let go, he would do the same with Indira ? I would give the benefit of doubt to Rishi as he was concentrating on Indu to save from Sameer all the time, as he very well knew the sleeping beast in Sameer has woken up with Indu and Rishi provocation and he could do anything and that is how even he cracked down that kidnap plan.
But Rishi couldn't second guess what was cooking between Sameer and Mandira. With Sameer's losing badly and all his plan being destroyed by Rishi, it was obvious the lid was about to blown away and Mandira didn't help the cause and dug her own grave by blackmailing and provocating him.
The moral of the story thereby for both Indu and Mandira is: "Agar saap ke bill mei haath daloogi toh kaatega hee na ?" Indu ne tho Rishi ka vaccination se baach gaye, par Madira paas gaye.'