It is wrong to act violently towards someone because they are sinners. Two wrongs don't make a right. We should teach people to not judge, and to love even their enemies. Of course, we should be careful that we are not understood as if we are defending the sin of homosexuality.
And, it is hard to tell someone about their sin. Perhaps, let them know what you believe and why you think homosexuality is a sin, but you can't force someone into believing like you. You can only offer Truth, facts, and pray that God will illumine them.
P.S. However, in the case of homosexuality, I wouldn't start with telling them that it is a sin. I'd start by telling them about Christ, The Gospel, and work it gradually from there, if possible. I mean really, our greatest problem is not our individual sins, but our lack of faith, and knowledge of God.