Having suicidal / homicidal thoughts... Would the ER take these thoughts seriously?

Yes they would take you seriously. I commend you on taking steps to seek treatment. If you believe that you are unable to commit to seeking treatment you can always take comfort in calling 911 and allowing emergency medical services to evaluate you and transport you to the hospital. Additionally, you if you would like to speak with a mental health professional about your thoughts you can call 1-800-273-8255 24 hours a day / 7 days a week.