the proof for $\chi(S) \leq 0$ starts as follows: note that $S \cong \mathbb{R}^2$ in this case. let $p: \tilde{S} \to S$ be the covering map. suppose $\tilde{\alpha}, \tilde{\beta}$ intersect in at least two points. then, there is an embedded disk $D_0$ in $\tilde{S}$ bounded by one subarc of each of the two lifts.
by compactness and transversality, the intersection $(p^{-1}(\alpha) \cup p^{-1}(\beta)) \cap D_0$ is a finite graph, if we think of the intersection points as vertices.
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compactness of what is used here? I don't see why this intersection should contain only finitely many …