Because of the pressure to redeem the Malfoy family name. Plus he had to kill Dumbledore and he couldn't do it because he knew deep down that he could never become a murderer. Plus he probably knew that Voldemort didn't expect him to succeed. He knew that he might be killed too and all of that just scared him and so he went into the bathroom to cry a lot (which is why Moaning Myrtle had a little crush on him).
Cause he was supposed to kill Dumbledore and he didn't succeed the first like 4 times and I guess there was a lot of pressure. He 说 Voldemort would kill him if he didn't do it. ALOT OF PRESSURE!
