What to say to atheist?

So I put a youtube video a while back using John Martinoni’s apologetic tactic, which is simply pointing out that without God, there is no objective reason that the holocaust (or any killing for that matter) is wrong. Most people replied in the comments trying to come with some objective reasons (which were really just their opinions), however someone replied that I was right; their is no objective reason reason that murder is wrong, however in his/her opinion it is wrong. And he/she also said that he/she can’t agree with something just because it would be detrimental to society if he/she didn’t. So basically, where do I go from here? What do I say now?
Here is a link to the video and comments in case you want more details: youtube.com/all_comments?v=xellBxSsug4 (you may not be able to see every comment that I referred to because I censor comments on most of my videos (to avoid cursing and potentially scandalous comments being shown publicly on controversial videos) and I have been having technical problems with being able to approve comments)

All I can say is that most people posting in that were using terrible reasoning… No one seems to be listening to your arguments, nor is anyone giving you a good answer, but as you said, man can be proud. All those other people are too proud, thinking they’re right, to say that they are wrong.

I like the argument, though I think it would be better suited for a one-on-one rather than an open format, because too many people talking and you keep back tracking.

If anyone of them had actual reason they would have to say, if they don’t believe in God, is that it isn’t objectively wrong, but because majority does not agree with the actions, that he shouldn’t be allowed to do it.

The closest person to the right answer said that it is because we are programmed with it, which I would agree with (Natural Law), but he failed at where it would have to have come from.

You can throw these at them:

Prove to me that God exists
  Long Version
  Short Version

Prove to me that Jesus Christ is the Messiah of the Bible

Prove to me that the Catholic Church is the Church that Jesus founded

Some more:This site has a “How to Talk to Atheists” Mp3 download:

shop.catholic.com/catholic-digital-media/audio-1/mp3-how-to-talk-to-atheists.html

This is a 5 DVD Catholic series helping college students with atheists:

magisreasonfaith.org/sscart/index.php?l=product_detail&p=4

This is a Catholic website on debating with atheists:

arguingwithatheists.com/Index.htm

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