It may not be self-defense unless your ex-boyfriend was, to a reasonable person, imminently--or immediately--threatening or instituting a physical attack. It is only self-defense when an attack is occuring or is about to occur. Future threats, vague oral/verbal threats, threats which are so over the top no one would believe them, or threats of non-violent actions (such as "I'll ruin you") do not justify drawing a weapon on someone.