Ich bringe sie um
Why does this translate to "I'll kill you"? When it is saying word for word "I bring them around".
'Umbringen' does in fact mean 'to murder'.
Often german words with prefixes can have different meanings than their root words. It's just how the german language works--any language, really. They all have their expressions and places where word-for-word translations don't always work.
Keep in mind that sometimes the words are truncated and rearranged, e.g.:
"Ich werde sie umbringen" => "Ich bringe sie um"
"ich werde umkehren" => "Ich kehre um"
"Es wird mir einfallen" => "Es fällt mit ein"
As Sara said for "umbringen", but one other note: "sie" [small 's'] means "her" or "them", not "you" [which would be "Sie"].
It's a bit like saying in English that you're going to "bump somebody off", or that somebody has "kicked the bucket". A non-English speaker would be completely confused by these colloquial expressions for killing and dying.