In Macbeth, how does killing Banquo connect to the killing of Macduff's family?

Concerning Shakepeare's Macbeth, the
ordering of killings demonstrates the extent to which Macbeth will go in order to
maintain and enhance his power--sort of. 

Ordering Banquo's
murder makes strategic sense for Macbeth, because Banquo knows about the witches'
predictions.  Banquo suspects Macbeth of murdering Duncan and Macbeth suspects he
suspects, so to speak.  Killing Banquo is a strategic move.  Ordering the death of
Fleance, however, demonstrates how Macbeth's ambition has grown.  Before killing Duncan,
all Macbeth dreamed of was being king.  He had no thoughts of creating a dynasty by
having his heirs continue as kings of Scotland.  Once he has the crown, though, it's no
longer enough.  He orders Fleance's death in an attempt to open the way for him to
create a dynasty.  He tries to enhance his power.

Ordering
the slaughter of Maduff's family, however, does not improve Macbeth's chances of holding
on to power, nor does it enhance his power.  It serves no strategic purpose.  Killing
Macduff would serve a strategic purpose, but slaughtering his family does not.  This
slaughter is more personal.  Macduff refuses to give loyalty to Macbeth,
snubbing Macbeth by not attending his coronation or his feast.  When Macbeth finds out
that Macduff has traveled to England to join Malcolm and seek help from the English, he
lashes out at Macduff by ordering the slaughter of his family.  Macbeth can't get
at Macduff, so he lashes out at his family instead.  This demonstrates, not Macbeth's
ambition or its growth, but his pettiness and ruthlessness and callousness.  It's almost
like Macbeth is throwing a sophisticated temper tantrum--very dangerous when we're
talking about a man with great power in his kingdom. 

In
short, then, the killings you ask about are connected in that a tyrant orders them. 
They also, by the way, reveal Macbeth doing his own planning (not involving his wife,
who is the better planner) since murdering Duncan.  But the murders are different in the
purposes they serve and by what motivates them.