You are framing this as a matter of personal ethics, rather than a business decision, so it's difficult to answer your question. You feel that Twitter did Tumblr and Instagram wrong. I tend to think that taking sides between two giants is dumb - the giants won't notice, but you'll get hurt.

If I had already built the Twitter functionality in, I would not preemptively rip it out, I would wait for them to shut that functionality off. Then I would raise a huge stink about it, get the name of the service in the news and maybe capture some Internet good will.

If I hadn't built Twitter functionality, I wouldn't spend any resources on building it until after launch.