Why do girls leave you on read?

"...someone doesnt have the respect or decency to at least reply"

This is where social media can be so dangerous, because she has as much right to ignore your DM as you did to send it. This is the part a lot of people don't get!   Sure, it's polite or decent to respond, but that doesn't mean it's impolite or disrespectful to NOT respond. 

You say you aren't taking this personally, but of course you are!  You feel like you've just "said" something to someone and they ignored you.   In real life, this would be very rude.  But a DM isn't real life. Another way to look at it:  If she had sent back an emoji response, would you feel better?  I'm sure you would.  But why?  Ask yourself that.

Not to sound like your mother lol, but treat this as a lesson learned.  When it comes to people at school, don't DM anyone until you've established a real life friendship or whatever.  With a good friend, if they read and don't respond, or they say something weird, you'd just go on with your day.  You can't do this if the response matters to you.

PS - Ignore "no mercy".   This person seems to spend their life on YA insulting anyone they can. 

it's either that she has no interest at all and some girls then will leave u on read, others however may do this to see if u will carry on the convo. but if u always hit them up and u see she doesn't have interest and yet still carry on then she will leave u on read.

she does not like u, she does not want u and she does not give even a slightest fck what u think of her, so she does not make an effort to be civil to u

If what you had to say was important, you would say it in person. She doesn't owe you her attention. Since you are clearly very fragile, she made the right choice. If she had rejected you, then you would have been ranting about how disrespectful she was for not giving you a chance. The problem is your ego, not her boundaries.