Well first of all... you definitely should have quaratined the new turtle because it could have had diseases and spread them to your RES. You are supposed to quarantine new turtles for about 3-4 weeks.
Also, i dont think it matters that the turtle was a different kind. if the new turtle was SMALLER than the res, he probably thought it was food and tried to eat it. This happens with everything that you put in the RES tank. I know that mine try to eat everything i put in there.
You probably should have tried to introduce them slowly (if the turtles are about the same size as each other)... but if there is a huge size difference, don't even try. Because the RES could definitely kill a new turtle.
The only way you could keep another turtle in with your RES is if you get a turtle of 'about' the same size and introduce them SLOWLY.
Plus, if you didnt have a big enough tank for the turtles, the RES could have been feeling territorial and tried to attack the other turtle because he was cutting into HIS territory.
And even through all of this, there is a chance that your turtle just doesnt want bothered with another turtle. I have seen PLENTY of posts that say "i lost my one turtle because my one RES killed the other one"... and sadly to say, sometimes turtles just can't live together.
If i were you, it sounds like your turtle doesnt want company. if he seems lonely, get him some toys. a bubble wand or a ping pong ball to swat around in the water. I don't think it's worth killing a turtle to find out if your RES is lonely!
But in the end, the choice is up to you....
now about the pointing of the shell. This could be caused by either a calcium definciency (which means you will need to somehow implement calcium into his diet.) You can either get a cuttle bone and cut it up for him to eat or you could get that calcium dust and sprinkle that on the pellets or the crickets.
this 'pyramiding' as they refer to it, could ALSO be caused by overfeeding your turtle. You should only be feeding your turtle enough pellets that will fit into his/her head about two - three times per week. All other times you should be offering him greens, such as collard greens, mustard greens and dandelion greens. And other such veggies or fruits (except SPINACH!)
hope this helps! 
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~ Evie 
Pets:
RIP 0.1.0 German Shepherd {Dusty}
0.1.0 German Shepherd/Collie mix {Shadoe}
0.0.2 RES turtles {Booger & Snot}
1.0.0 African Clawed Frog {Bingo}
0.0.2 Snail {Gary & Larry}