
Tsundere means you give off a coarse exterior but inside you are soft and cuddly. Like someone who picks on the one they like? But not in a sadistic way...that's something else. Basically, you act like a jerk but mainly because you are too sensitive and care too much.
Yandere is the opposite. It's basically giving off the appearance of a kind person but is really a psycho underneath. Like 'mr. nice guy' types who are stalkers and murders. It's like a switch is flicked and out comes a serial killer. Even if they aren't really a murder you believe that they could be.

Ah, yeah. Damn, it's still in my to read/to watch list. I should move it up.
https://duckduckgo.com/?q=yun+gasai&t=ffcm&iax=1&ia=images
ohh that face.... he's qualified to bcome a yandere... lolz