How can the character of Macbeth be modernized to suit the 21st century?
Macbeth by William Shakespeare is set in a quite different culture than the contemporary western one. The characters are formed by their environment and the plot constructed around incidents and belief systems quite unlike modern western ones. Thus you cannot actually "modernize" the character of Macbeth. You could simply invent another character with a limited number of similarities, but it would not be a modern Macbeth, simply a different character.
If you wanted a 21st century character who could murder a king to become an absolute monarch, you could set the play in the Middle East or sub-Saharan Africa, where a few absolute monarchies still exist, but there might be some difficulty in transposing other elements of the play such as the role of Lady Macbeth (the majority of 21st century absolute monarchies are also polygamous). Witches would fit in an animist setting in Africa, but not in an Islamic one.
There is no reason, however, that the character of Macbeth needs to be changed to "suit" 21st century audiences. Part of the point of experiencing literature is to learn about cultures of other times and places. Modernizing earlier works means that instead of learning about different cultures, you just end up encasing yourself in the narrow bubble of your own culture.