Why did God do nothing to Abraham when he had a concubine and to Solomon who had multiple concubines?
God has, from the very beginning, intended that marriage be between a single man and a single woman for life. This is clearly the pattern described in Genesis 2. Yet, because of their hard hearts, God allowed certain behaviors for the Jews that he does not support at all. This is proved by Matthew 19:1-9. Here Jesus said that God allowed divorce, but not because he approved of it. In fact, God has always hated divorce (Malachi 2:16). However, because of the gross cultural sin in the ancient world, and because of the hard hearts of the Jewish people, God allowed for divorce. He also allowed, but certainly never approved, polygamy and concubinage. It is a fact that literally every single example of polygamy and concubinage in the Old Testament led to disaster, or at the very least, great marital unrest. God opposed this behavior by example, rather than by direct commandment in the Old Testament. He made the desired pattern clear, but for practical reasons and because of their hardness of heart, he allowed behavior to the Jews he abhors. Look what happened to Solomon!
A similar example is slavery. Any believer who knows God and the Bible will agree that God does not support slavery. No way! Yet, in the Old Testament, God allowed for a very limited form of slavery, in which abuse was not allowed, and all slaves were eventually set free. God allowed Solomon to do what he did, but look where it led him!
When Jesus told people that if their eye or hand caused them to sin they should pluck it out or cut it off, he was using hyperbole. Hyperbole, by definition is purposeful exaggeration used in order to make a point. That is exactly what Jesus does when he tells us to pluck out our eye if it causes us to sin. I guarantee you that none of the apostles took him literally in this. To say that he did (as some Hindu skeptics do) is to be dishonest with the text. If a Hindu says that Jesus was supporting sinful behavior they are not being honest with the text. This is simply not an honest way of analyzing what Jesus said. Is there any example of a Christian taking him literally? I know of none. Nevertheless, what Jesus said is true. It would be better to enter heaven with one eye that not have two eyes and go to hell. It would be better to have our eye plucked out than to go to hell. However, surely there are better means to stop lusting than to pluck out our eyes. What about repenting and stopping this sin. Similarly with having a hand cut off. The fact is that it really would be better to lose a hand and go to heaven than to keep both hands and go to everlasting destruction. Despite this, I am sure Jesus would prefer that we simply stop sinning with our hands than cut a hand off. Anyone who says that Jesus is proposing the cutting off of hands is being dishonest.
As for Hindu gods doing sinful things, of course this is true, but that is for Hindus to deal with. Hinduism is a polytheistic religion and much of what the “gods” do is sinful by a Christian standard of ethics. It is Hindus, not us, who need to answer for this problem. If you want to convert Hindus, I advise you point them to what is good in the God of the Bible, and to the evidence for the inspiration of the Bible and not put a lot of energy into proving how “bad” Hinduism is.
Why do people believe in this religion, despite the blatant sinfulness of these supposed gods? You should ask the Hindus themselves. Ask your Hindu friend, “What is the reason you believe in this religion?” Is it because of the evidence that your Scripture is inspired? Are you attracted to the morality of Hinduism? Is it because from Hinduism you learn that you are a god? It is very likely that the reason they give will not be the actual reason, as many people, (including Christians) are not self-aware as to why they do certain things. I believe that Hindus hold on to their religion for cultural reasons, largely. Christianity is a very demanding religion. Hinduism is not (at least not for the majority who are not monks). Why do people believe in atheism? Is there any historical record behind atheism? Why does anyone believe in any religion? This is a very complicated question with many answers. I could write a ten-page essay on the reasons that people believe in religions. All that really matters is that they do.