I agree with the previous comment on the wand "shapeshifting" to adapt to the wizard. Remember that Wands are ALMOST a living thing, and are very much sentient.
I know there is no confirmation on this and its is a widely believed theory.
This would clear why some "evil looking" are even so. I mean I doubt Ollivander would make a wand so "scary looking" like Voldemort's. It would seem fit that as the wizards comes to their own personality, the wand changes just like it can change or keep its allegiance.
Harry's is very natural looking, the handle is basically a bare trunk. I believe this reflects harry natural persona. He doesn't fake it, he says it how it is, but he is also pretty new to the magical world, not extremely fancy or concerned with etiquette but more as raw power and abilities
Bellatrix's has a bent and twisted shape, the way of holding it is twisted, with a straight one she'd be pointing at anything but her target. This is a clear parallelism to her mind status, twisted, bent, unsettling.
Voldemort's is dead looking, it has lost color and its "lively look" this too reflects him.
Severus' wand is black, straight, with a clear handle and "blade" so to speak. Everything was pretty cut and dry with it. You could say the wand is scary looking, but it is also plain and simple so it also be a good guy's wand. Just like Snape, his personality is also cut and dry. He has a feeling and acts accordingly. Like his wand his nature (good or evil) is not truly discernible and as a skilled Occlumency Practicer this is very fitting.
Umbridge's wand is very fancy, very ornamented, but, not at all ergonomic, not comfortable.
Whereas Draco's remained like the original, I believe this could indicate that Draco hasn't settled into his personality yet, we can see in the deathly hallows, that he starts helping Harry, questioning his allegiances. I believe Draco is a very malleable guy who hasn't had the change to develop HIS personality, but rather borrowed from his father.
Also Lucius' replacement wand was very plain, just like if the wand had had no time to "feel" Lucius and shape accordingly.
THIS IS NOT CANON, I know, but it makes darn good sense!