What happens if we rewrite the snippets with the other connector, keeping the cause and effect the same?
The original: He tapped his own life force because he wanted me dead.
The rewrite: He wanted me dead so he tapped his own life force.
The original snippet places the most emphasis on the effect ("he tapped his own life force"), whereas our rewrite emphasises the reason ("he wanted me dead").
How about the other one?
The original: He could poke it from his side so it would fall out on the other side.
The rewrite: It would fall out on the other side because he could poke it from his side.
Again we've shifted the emphasis, this time from the cause ("poking it from his side") to the effect ("the key falling out on the other side").
Which way you use will depend on what's most important, or whatever sounds best to you.