First of all, we don't use conjunctions in informal speech, almost entirely, except and / but when necessary. So, to move it one step closer to our native style, try the following. Also we drop obvious descriptions (from him) and use contractions (he's):
"When I see him, I walk away-- he's a rogue."
That's our native speaker style -- pared down.
To really make it AmE "youth" style, change the consequence to a mere emotional attitude, introduced by "like." Also, exaggerate and overstate the reason with a demonstrative: "such a rogue."
"When I see him, I'm like, 'let's get out of here.' He's such a rogue."