Why does Malcolm appear weak?

Malcolm can certainly be seen as wary and cautious, but that does not mean that he is weak. After his father, King Duncan, is murdered by Macbeth, there is this exchange between Malcolm and his brother, Donaldbain:

MALCOLM:

[Aside to Donalbain.] Why do we hold our

tongues,

That most may claim this argument for ours?

DONALBAIN:

[Aside to Malcolm.] What should be spoken here,

where our fate,

Hid in an auger-hole, may rush and seize us?

Let's away;

Our tears are not yet brew'd.

Malcolm actually seems ready to act against whomever it was that killed their father, but Donaldbain ecourages him to act with caution.

Donalbain goes off to Ireland and will not appear again in the play. Malcolm goes to England where he will tell his tale and amass an army to fight Macbeth. Nothing weak can be seen in those actions.

Then in Act 4, scene 3, Macduff goes to visit Malcolm in England. Malcolm doesn't know if Macduff is a freind or a foe, so he is rightfully cautious in dealing with his visitor:

MALCOLM:

What I believe, I'll wail;

What know, believe; and what I can redress,

As I shall find the time to friend, I will.

What you have spoke, it may be so perchance.

This tyrant, whose sole name blisters our tongues,

Was once thought honest. You have loved him well;

He hath not touch'd you yet. I am young, but something

You may deserve of him through me, and wisdom

To offer up a weak, poor, innocent lamb

To appease an angry god.

This is prudence, not weakness. And when talking about the war to come, this does not sound like weakness:

MALCOLM:

Be't their comfort

We are coming thither. Gracious England hath

Lent us good Siward and ten thousand men;

An older and a better soldier none

That Christendom gives out.

And later in the scene, when Macduff learns that his family has been murdered, Malcolm says to Macduff:

Merciful heaven!

What, man! Ne'er pull your hat upon your brows;

Give sorrow words. The grief that does not speak

Whispers the o'er fraught heart, and bids it break.

And in Act 5, scene 4, he leads the charge on Macbeth's castle:

MALCOLM:

Let every soldier hew him down a bough,

And bear't before him: thereby shall we shadow

The numbers of our host, and make discovery

Err in report of us.

Lastly, Malcolm has the final words of the play. All is moderation and strength, attributes befitting the new king of Scotland:

We shall not spend a large expense of time(

Before we reckon with your several loves,

And make us even with you. My thanes and kinsmen,

Henceforth be Earls, the first that ever Scotland

In such an honor named. What's more to do,

Which would be planted newly with the time,

As calling home our exiled friends abroad

That fled the snares of watchful tyranny,

Producing forth the cruel ministers

Of this dead butcher and his fiend-like queen,

Who, as ’tis thought, by self and violent hands(80)

Took off her life; this, and what needful else

That calls upon us, by the grace of Grace

We will perform in measure, time, and place,

So thanks to all at once and to each one,

Whom we invite to see us crown'd at Scone.