Finished reading The Idiot by Dostoevsky 3 days ago and wtf I have read crime and punishment, brothers karamazov, notes from underground and now finished The Idiot but I don't understand it completely. 

First of all the tragic end in such less pages makes me wonder why would Dostoevsky would that. In middle part some pages felt like drag like the Ippolit suicide speech. The last 50 pages have more drama and development than the middle 100-150. I don't understand why would Dostoevsky would do that. The brothers Karamazov jumped like this too but it's ending was still much more detailed than this. 

It felt like had my heart stabbed when I realised Nastasya was killed. Why didn't Prince wanted her to be taken away after she was dead? I still don't understand what Dostoevsky was trying to say clearly. For me it's just a tragic end of a pure hearted man. Was Dostoevsky trying to say a pure hearted man like an idiot will not be able to survive in the modern world? 
Please help.