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I think you can get a much faster (and maybe easier...) proof using Riemannian geometry, as follows:

First, recall that a semi-simple connected Lie group $G$ is compact if and only if its Killing form $B$ is negative-definite (the proof is easy, see, e.g., Thm 2.28 in these notes). The implication you need ($G$ compact semi-simple $\Rightarrow$ $B$ neg.-def.) actually follows directly from $B(X,X)=tr(ad(X)\cdot ad(X))$ using an orthonormal basis with respect to an auxiliary bi-invariant metric to compute this trace.

The Ricci curvature of any bi-invariant metric on $G$ (that exists because $G$ is compact) can be computed as: $$Ric(X,Y)=-\frac14 B(X,Y),$$ see Remark 2.27 in the same notes. By the first observation above, since $G$ is compact and semi-simple, its Killing form $B$ is negative-definite. Hence the above formula gives $Ric>0$. So, by the Bonnet-Myers Theorem, $G$ must have finite fundamental group. Q.E.D.

Perhaps this was the sketch of proof originally suggested to the OP?

I think you can get a much faster (and maybe easier...) proof using Riemannian geometry, as follows:

First, recall that a semi-simple connected Lie group $G$ is compact if and only if its Killing form $B$ is negative-definite (the proof is easy, see, e.g., Thm 2.28 in these notes). The side we will use ($G$ compact semi-simple $\Rightarrow$ $B$ neg.-def.) actually follows directly from $B(X,X)=tr(ad(X)\cdot ad(X))$ using an orthonormal basis with respect to an auxiliary bi-invariant metric to compute this trace.

Now, the Ricci curvature of any bi-invariant metric on $G$ (that exists because $G$ is compact) can be computed as: $$Ric(X,Y)=-\frac14 B(X,Y),$$ see Remark 2.27 in the same notes. By the observation above, since $G$ is compact and semi-simple, its Killing form $B$ is negative-definite. Hence the above formula gives $Ric>0$. So, by the Bonnet-Myers Theorem, $G$ must have finite fundamental group. Q.E.D.

I think you can get a much faster proof using basic Riemannian geometry (maybe this was the sketch of proof originally suggested to the OP?) as follows:

First, recall that a semi-simple connected Lie group $G$ is compact if and only if its Killing form $B$ is negative-definite (the proof is quite simple, see, e.g., Thm 2.28 in these notes).

Since you start with a compact semi-simple Lie group $G$, by compactness, it has a bi-invariant metric. The Ricci curvature of this metric can be computed as:  $$Ric(X,Y)=-\frac14 B(X,Y),$$ see Remark 2.27 in the same notes. By the first observation, since $G$ is compact, its Killing form $B$ is negative-definite. Hence the above formula gives $Ric>0$. Finally, by the Bonnet-Myers Theorem, $G$ must have finite fundamental group.

I think you can get a much faster (and maybe easier...) proof using Riemannian geometry, as follows:

First, recall that a semi-simple connected Lie group $G$ is compact if and only if its Killing form $B$ is negative-definite (the proof is easy, see, e.g., Thm 2.28 in these notes). The implication you need ($G$ compact semi-simple $\Rightarrow$ $B$ neg.-def.) actually follows directly from $B(X,X)=tr(ad(X)\cdot ad(X))$ using an orthonormal basis with respect to an auxiliary bi-invariant metric to compute this trace. 

The Ricci curvature of any bi-invariant metric on $G$ (that exists because $G$ is compact) can be computed as:  $$Ric(X,Y)=-\frac14 B(X,Y),$$ see Remark 2.27 in the same notes. By the first observation above, since $G$ is compact and semi-simple, its Killing form $B$ is negative-definite. Hence the above formula gives $Ric>0$. So, by the Bonnet-Myers Theorem, $G$ must have finite fundamental group. Q.E.D.

Perhaps this was the sketch of proof originally suggested to the OP?

I think you can do a much faster proof using basic Riemannian geometry as follows. Let $G$ be a semi-simple connected Lie group. Then $G$ is compact if and only if its Killing form $B$ is negative-definite (see, e.g., Thm 2.28 in these notes). Since $G$ is compact, it has a bi-invariant metric, and the Ricci curvature of this metric is $Ric(X,Y)=-\frac14 B(X,Y)$ (see Remark 2.27 in the same notes). Since $B$ is negative-definite, it follows that $Ric>0$, so by the Bonnet-Myers Theorem, $G$ must have finite fundamental group.

I think you can get a much faster proof using basic Riemannian geometry (maybe this was the sketch of proof originally suggested to the OP?) as follows:

First, recall that a semi-simple connected Lie group $G$ is compact if and only if its Killing form $B$ is negative-definite (the proof is quite simple, see, e.g., Thm 2.28 in these notes). 

Since you start with a compact semi-simple Lie group $G$, by compactness, it has a bi-invariant metric. The Ricci curvature of this metric can be computed as: $$Ric(X,Y)=-\frac14 B(X,Y),$$ see Remark 2.27 in the same notes. By the first observation, since $G$ is compact, its Killing form $B$ is negative-definite. Hence the above formula gives $Ric>0$. Finally, by the Bonnet-Myers Theorem, $G$ must have finite fundamental group.