Roots!!!

Hello everyone,

I have removed a conifer tree that had died from my garden. Wasn't sure why it had died as the other 4 seem to be ok. I thought the cause could have been from my neighbours plant. (although I have no idea if plants can kill another plant) I don't know what it is. I have put a few pics up but I don't know if that will help.
I want to plant a mahonia where I removed the tree from and have just found quite long roots which I presume are coming from the neighbours plant. The roots have a red colour to them.
Now I'm thinking that the roots killed my poor conifer and am hesitant to put the mahonia in its place.
Can any of you knowledgeable people help a mere beginner

 

May we have some better pics Nina, please.:gnthb: