Peor vs. más malo
In Spanish, peor and más malo can both seem to mean worse, but they are not interchangeable. Peor is the standard, correct comparative for bad, while más malo is used in much more specific contexts, primarily related to moral character.
Peor
A2Peor is the direct equivalent of worse. It is the standard, irregular comparative form of the adjective malo (bad). You should use peor in the vast majority of situations when comparing the quality, condition, or state of things, people, or situations.
La película de anoche fue peor que esta.
(Last night's movie was worse than this one.)
El tráfico está peor que nunca por la lluvia.
(The traffic is worse than ever because of the rain.)
Me siento peor hoy que ayer.
(I feel worse today than yesterday.)
Este remedio es peor que la enfermedad.
(This remedy is worse than the disease.)
Su segunda novela es considerablemente peor que la primera.
(His second novel is considerably worse than the first one.)
El servicio en este hotel es mucho peor de lo que esperaba.
(The service at this hotel is much worse than I expected.)
Más malo
B2Más malo literally translates to more bad. Its correct use is very specific and limited. It is used to describe a person's character as being more evil, meaner, or more wicked. Using it to describe the quality of an object is generally considered incorrect or highly colloquial, as peor is the proper term.
El villano de esta secuela es más malo que el original.
(The villain in this sequel is more evil than the original one.)
Algunos creen que el rencor te hace una persona más mala.
(Some believe that holding a grudge makes you a meaner person.)
Mi perro es tranquilo, pero el de mi vecino es más malo y siempre ladra.
(My dog is calm, but my neighbor's is meaner and always barks.)
En el cuento, el lobo es más malo que la bruja.
(In the story, the wolf is more evil than the witch.)
Desde que es el jefe, se ha vuelto más malo con los empleados.
(Since he became the boss, he has become meaner to the employees.)
Ese niño es más malo que el otro; siempre está molestando.
(That child is naughtier than the other one; he is always bothering people.)
Summary
To summarize, always use peor as the general word for worse when comparing the quality or state of nearly anything (food, movies, weather, health, situations). You should only use más malo in the specific context of describing a person or character as being more evil, meaner, or more wicked.