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 press A immediately afterward. 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.