Why are ravens seen as bad omens?

Well, there are a variety of reasons that one could offer. Primarily, they're associated with misfortune because of their appearance (dark, sometimes eerie), croaking, and dietary habits. Eating the dead remains of persons isn't a reflection of good character, after all. So, given their nature, one might assume that the sight of a raven means the dead are nearby. Over time, superstition and lore has given greater meaning to their supposedly cursed name. Overall, their primary association would be death (though sin itself seems to also be a common trend). I would look into purchasing a book or investing in a bit more research if you're still curious of more, as I've only divulged the basics.

Ravens are black and they are also carrion feeders. The ravens of the Tower of London no doubt fed from the gibbets of executed prisoners. From earliest times the association must have been made that they were connected with death and therefore to some extent unwelcome.

It probably has to do with their color: All Black. This is not supposed to be a racist comment at all. But if you go back to the era that these myths were created and whatnot. Even today you still see signs of it. Like witches are always seen in black with their black hate. Death is always pictured in a black robe of sorts. So black kind of means evil or bad in a way, according to our past cultures at least.

"A lot of negative raven symbolism comes about from their appearance on battlefields. They are scavengers (and curious to a fault), and are often seen picking at mangled remains of fallen warriors on battle grounds."

See more below

http://www.whats-your-sign.com/raven-symbolism.htm...

Ravens were the birds associated with battle and death in tribal Europe. They were the eyes of The Morrigan, the goddess of battle of the Celts. Ravens appear where there is death, they are scavengers.

not to be picky here - but a the correct name for a large grouping od raven is not a murder.

you have a murder of crows

you have an unkindness of ravens or a conspiracy of ravens..

The raven is the most cherished creature to me for so many reasons.

You need to understand that Crows and Magpies use to follow certain armies into battle for after all Crows and Magpie's are mini-Vultures and they knew that following those soldiers into battle will results into a smorgasbord that would feed those birds for days...................

I agree with Austin and it could also be because many people were afraid of monsters coming out at night and night is black?

Because their black and black suppose to represent darkness and death in folklore. Tho black is not actually a color