Loathsome expresses strong negative feelings; it's much more intense than just 'dislike' or 'disgust'. People found his deceitful actions absolutely loathsome.
Pro Tip 2/3
Intensity Matters
Use 'loathsome' for the most extreme cases where something causes deep revulsion. The crime scene was described as particularly loathsome.
Pro Tip 3/3
Sensory Disgust
Loathsome is often used to describe things that repulse the senses, like taste or smell. The loathsome taste of spoiled milk made me gag.