Understanding「そうなるとやはり犯罪者かしかしあまり大物でもマズい」

Question

From the first episode of Death Note, I'm having trouble understanding

そうなると やはり犯罪者か しかし あまり大物でもマズい

1. Why is there a と after そうなる?
2. Is やはり's reading "矢張り"?
3. Is あまり's reading "余り"?
4. What does this sentence mean?

Answer 1

Why is there a と after そうなる?

そうなると is a set expression meaning "if that is the case", "if it becomes so", or "when it becomes so".

Is やはり's reading "矢張り"?

Yes, this is 矢張り but it's almost always written with kana alone. やっぱり is also commonly used but it's more informal. Additional details can be found on this post.

Is あまり's reading "余り"?

Yes, あまり is 余り.

What does this sentence mean?

Here is my translation (disclaimer: I'm not a native speaker):

If that's the case, it should be a criminal. But someone too important would be bad.

I also looked up the official translation from the English subtitles which is the following:

In which case, the victim should probably be a criminal. But it can't be someone too infamous.