Why is it so frightening for Haymitch to be Peeta and Katniss sponsor in his drunken condition?

I wouldn't say it was frightening. He wasn't very motivated to do it. Once he won the games years ago, he treated into his own world and didn't expect any real responsibility. Plus he didn't think he would have real victors or guys that would stand a chance in the games to care. This mindset would soon change when he gets to know Peeta and Katniss.