With an army.
Malcolm plans to gather an army to challenge Macbeth's rule and reclaim the throne of Scotland. He aims to use a combination of military force and strategic alliances with other nobles to weaken Macbeth's position and ultimately overthrow him. Additionally, Malcolm seeks to gain public support by portraying himself as the rightful heir and a just ruler compared to Macbeth.
Macduff and Malcolm plan to overthrow Macbeth in England.
Both Malcolm and Macduff believe Macbeth is a tyrant to be overthrown. However, Malcolm is a tyrant himself, if not even more repulsive than Macbeth. Though Macduff is not fully aware of this fact. Malcolm manipulates Macduff into slaying Macbeth and then plea allegiance to himself as the King of Scotland.
The English king, Edward the Confessor, has offered to provide troops and support to help defeat Macbeth and restore Malcolm to the throne of Scotland. This assistance comes after Malcolm sought refuge in England following the murder of his father, King Duncan, by Macbeth.
If by Macbeth's murderer you mean Macduff (although it wasn't murder because he killed him in a fair fight) he aligned with Malcolm the elder sun of King Duncan.
Young Siward is a Scottish nobleman and a soldier who fights alongside Malcolm to defeat Macbeth. He is killed by Macbeth in combat during the battle.
Macbeth does not believe that Malcolm cannot harm himself. He is quite prepared to believe that Malcolm might harm Malcolm, which is what "harm himself" means. Macbeth does believe that Malcolm cannot harm him (i.e. Macbeth) because Malcolm's mother was apparently a woman and the witches said that none of woman born could harm Macbeth.
In the play Macbeth, Malcolm portrays himself as a potential sinful tyrant in order to to test Macbeth. Malcolm wanted to see just how loyal Macbeth would be.
In Macbeth? King Duncan names his son Malcolm The Prince of Cumberland who is the heir apparent. After his murder, Malcolm flees so Macbeth is named to be King but later on in the play Malcolm, Duncan's son, becomes king.
England.
At the end of Shakespeare's play Macbeth; Malcolm, son of Duncan is proclaimed to be King of Scots. (In actual history, Macbeth was succeeded by his son Lulach; who was then was assassinated and deposed by Malcolm)
Macbeth views Malcolm as a stepping stone to his own ambition - to eliminate Malcolm and claim the throne for himself. Calling Malcolm a "step" signifies Macbeth's belief that he must overcome and overthrow Malcolm in order to achieve his goals. It reflects Macbeth's ruthless and ambitious nature.
Macbeth was compared to the devil