Messages Crashes When Opening Message

Use a program like iBackupBot to backup the messages first. Then use it to delete them all directly from your iPhone. Try it now. Problem gone ? It was one of your messages then.

Problem still there ? Then create a backup in iTunes and do a restore to factory settings (turn the iPhone off, hold the homebutton and plug it into your computer (while still holding the homebutton). Once it shows the iTunes icon on your iPhone you can let the homebutton go. iTunes will detect an iPhone in recovery mode. Choose to repair it when iTunes prompt you.

Afterwards restore your backup. Problem gone ? Then it was a software issue.