I don’t know what country you are in, if you are in the USA not likely you would get arrested for that. Police don’t like to waste their time with password stealing or deleting someone account type stuff. They really have real crime to deal with, not kids pressing charges because someone deleted a social web account.

Depending on your jurisdiction, there are different law in regards to "hacking" an account, and yes accessing an account even though you know the Username and Password is hacking an account as you are doing it without their knowledge. You best bet is to let your former friend live their life, and if they are too stupid to give you that information in the first place and then too stupid to change the password afterwards you are better off without them in your life, and judging from the actions you wish to carry out, they are better off without you in their life as well.

If you were to do this stupid move, opps guess what Yahoo logged your IP address and login information and from there information can be gathered from your local police department in case the person account you were going to delete was to press charges, and opps again asking the question show premeditated thought into doing this, so even if there is not enough evidence to charge, there is more then enough evidence to sue you back into the stone age.

Also if you are thinking that deleting this post will save you, you are wrong as it maybe gone from the site, but all the information exchanged is still stored on Yahoo’s side of things, as just remember people may forget, the internet NEVER does.

Ronin is correct. You don’t own the account, leave it alone.

For all you know, that could be a bait account created to attract criminals so they can be tracked more easily by law enforcement. Do you want to get caught up in such an investigation ?