Is Battle of the Immortals safe to download and install?

Seeing as there is a large community for that game it's unlikely it's purely a malware. About it containing a trojan - reading the content of this forum thread and this post suggests it's a false positive by a specific security software (or two).

On the other hand, the poster from your provided link was pretty convinced that the game is up to no good.

I don't know the credentials of any of these people; but as the game does look serious and has some players, I doubt it contains a trojan (a back door into your machine). That's just my personal opinion, though. Services like Virustotal can help you see which anti-virus software flags a file as harmful and think for yourself if it looks like a false positive or not.