Hm… How else do I explain this… another way of translating ~なら is ‘if ~’ i.e. ‘if ~ is true’. Also, どうせ comes from どう and せよ, which is the written imperative of する, and is often used for hypothetical things, like いずれにせよ (roughly ‘whichever (one) choses’, from the structure ‘~にする’, which means ‘to choose/decide on ~’). Therefore, どうせ literally translates to ‘however (one) does i.e. acts’. That might help you understand, along with parsing the sentence as I said: [どうせ死ぬ]→なら. That is, translate どうせ死ぬ first, then add the meaning of なら.
Here are two possible translations of everything up to なら if you don’t quite see what I’m getting at:
‘If (he) will die no matter what,…’[/spoiler] OR [spoiler]‘In the case that he will die no matter what,…’