Murder Beam is the combination of all beams. It deals damage in a column over time (this is disputed), has the same damage type as Charge Beam dealing the same amount of damage as Power Bombs (i.e., 200), and can freeze enemies.
Murder Beam has viability against bosses, with the most prominent being Mother Brain's second phase and third phase as it will persist during the cutscene between those phases. That being said, it is inferior for real-time due to causing a tremendous amount of lag and being slower in damage output than charged Ice Beam/Wave Beam/Plasma Beam.
To equip Spazer and Plasma Beam at the same time, open the inventory screen, equip Spazer, and move the marker to any of the Boots items. Then, press Left and A on the same frame. Plasma Beam should then become equipped alongside Spazer with "VAR" next to it.
Murder Beam can only be fired when fully charged and facing left. Firing in another direction and/or firing an uncharged shot will cause the game to crash. To prevent the game from crashing, a charge should be initiated before entering the pause screen to enable this beam.
Alternatively, after equipping Murder Beam if you have missiles remaining you can auto-cancel one, and hold down the Shot button after firing it. This will allow for firing only charged shots.