If an atheist has always done good deeds during his life (abided by God´s rules preached by all religions), where would God (if there is any) allocate him after his death -- heaven or hell?

I personally believe that God, being so gracious as we have been told, will place him in heaven as it would be absurd to send him to hell where the likes of serial killers, etc. reside. So, if this is the case, why should anyone follow any religion? Just be good during your life (see no evil, do no evil and think no evil) and you will be placed in heaven even though you do not believe in God.

I am sure God will forgive an honest non-believer. Wouldn´t he?

There are several problems with your point. An atheist, by definition, is a person who doesn't believe there is a God. Therefore, he won't be concerned as to where God might send him as he doesn't believe God exists, nor would he feel bound to follow God's laws. The odds of an atheists perfectly keeping God's laws is absolute zero.

Second, though doubting the existence of God, you express confidence that you know precisely what God would and would not accept. And what forms the basis of your confidence? Why your very own opinion! I'm sorry to say, but as an individual human being, you are a very poor source for the thoughts and opinions of God. "For what man knows the things of a man except the spirit of the man which is in him? Even so no one knows the things of God except the Spirit of God" (I Corinthians 2:11). If you want to know what God will do with the atheist, you really need to ask God because only God knows His own mind.

Third, you think that salvation can be achieved by perfect law keeping. Paul addressed this issue at the beginning of Romans. In Romans chapter 1, starting at verse 18, he examines the Gentile societies as a whole and found that they were heavily involved in sin. In chapter 2 he examines the Jewish society and finds sin just as prevalent. He concludes, "What then? Are we better than they? Not at all. For we have previously charged both Jews and Greeks that they are all under sin. ... for all have sinned and fall short of the glory of God" (Romans 3:9, 23). The problem is that given a set of laws, man eventually breaks the law. Thousands of years of history demonstrates this consistent pattern of behavior. Perfect law keeping cannot save anyone.

Fourth, you ignore the spiritual aspects of what God requires. "But without faith it is impossible to please Him, for he who comes to God must believe that He is, and that He is a rewarder of those who diligently seek Him" (Hebrews 11:6). This statement alone shows us that an atheist cannot be saved by the very definition of atheism.

Finally, you attempt to rate sins. You find it easy to imagine that serial killers are going to hell, but what about the adulterer? Or the person who steals? Or the person who hides behind lies? "But the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death" (Revelation 21:8). Notice that God states that the unbelieving, that is the atheist, will end up in hell along with murderers and other sinners. What you are attempting to do is subdivide God's law and say that some parts are worth keeping, but others can be ignored in your opinion. James says it cannot be done. "For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all. For He who said, "Do not commit adultery," also said, "Do not murder." Now if you do not commit adultery, but you do murder, you have become a transgressor of the law. So speak and so do as those who will be judged by the law of liberty" (James 2:10-12).

See also:

Questions and Answers regarding Atheism
Questions and Answers regarding Salvation