Okay, I'm going to act like a good little beta should here and help you with characterization. :)
I, personally, would have trouble believing this in a plotline. Even if he was threatened by Voldemort, or another Death Eater, I don't believe he'd kill her. Draco is very attached to his mother, as was proved in GOF and HBP. (Please check "Spinner's End" for references.) If this occured, I would assume Draco would help his mother escape and act as if he killed her. (Not entirely sure if this would work as Voldemort has some way of knowing everything but he seems like the type that would try.) I myself could only see Draco harming his mother if under the Imperious Curse.
Draco kills his mother, sorry, it's part of the plot...
Hopefully this is set after fifth year because if it isn't, Draco wouldn't have learned how to make Polyjuice Potion. Unless his family owned a copy of "Moste Potent Potions."
...and he has taken Polyjuice Potion to hide on the train...
How? Did the Polyjuice Potion wear off on the train, because unless this happened, or he was completely out of character for the student he'd impersonated I don't think he'd be caught. You have to remember that Draco isn't stupid, I'm sure he'd find a way to keep the student he impersonated from running in and blowing his cover.
...He is discovered, caught and his dastardly plot exposed...
How would Harry get a hold of such truth serum? I would think only professors and other adult wizards would be able to brew it. Unless Hermione did it for him of course, but why would he need it in the first place?
...I was thinking that Harry could slip him something to ensure that he told the truth, but I wanted him to be so enraged with what happened that he just spilled his guts to the entire group...
Since Draco is a very head-strong, stubborn young man, do you really think he'd just spill his story out in rage? I think he'd try to keep it quiet as long as possible. Harry and his friends would be hard-pressed to get THAT kind of secret out of him; especially since he's never given into peer pressure from them before. He just dishes it right back. I believe he'd do the same in this circumstance both to protect himself (as a Slytherin) and those he works for.
Ah. So this is the cause of Draco's murderous attitude? Well I suppose we have to look into Narcissa's character a bit then don't we? (If you want this to remain canon) If you've read "Spinner's End" then you'd know that Narcissa's character is developed a lot in that chapter. In that chapter she is described as a loving mother - with a few faults - who holds nothing but affection and love for her son. She goes to the person she trusts most to protect her son because she wants what's best for him - life. Does that sound like a mother who would sell her son as a prize for Pansy's bidding?
...He didn't want to marry Pansy, and he was furious at his mother for offering him as a prize if Pansy did her dirty work...
You also have to think about Draco as well. Being the strong, stubborn, pampered Slytherin he is, do you think he's sit back and let Pansy marry him? I believe he'd only settle to an arranged marriage if it was what Narcissa truly wanted and thought would be best for him. I also don't think he'd go down without a fight if he really loathed her. ;)
Good. I too think Draco would enjoy being the magnet of women's affection. And yet, he would keep his feet firmly planted to the ground about what he wants - without being too egotisical over it. ;)
...I was thinking along the lines that while Draco enjoyed Pansy's simpering and doting, that he would never truly *want* to marry her...
^^ Just a few things to keep in mind while writing Draco :D
Hope I helped, good luck!
Fantastic post! :) 10 points to Slyth