diff --git a/cmd/acb-evolver/internal/arena/arena.go b/cmd/acb-evolver/internal/arena/arena.go
index 7fdbcb0..a4eccbf 100644
--- a/cmd/acb-evolver/internal/arena/arena.go
+++ b/cmd/acb-evolver/internal/arena/arena.go
@@ -61,6 +61,7 @@ type MatchOutcome struct {
 	Winner        int // 0=player0, 1=player1, -1=draw
 	Scores        []int
 	Turns         int
+	CombatDeaths  []int // bots killed in combat per player (kills credited to killer)
 	Err           error
 }
 
@@ -85,6 +86,11 @@ type Result struct {
 	Draws  int
 	Errors int
 
+	// Kill statistics across all matches (errors excluded).
+	TotalKills   int    // total kills credited to candidate
+	TotalMatches int    // non-error matches for kill rate normalization
+	KillRate     float64 // kills per match
+
 	// OpponentWinRates maps opponent BotID → candidate win rate vs that bot.
 	OpponentWinRates map[string]float64
 
@@ -195,6 +201,25 @@ func (a *Arena) Run(ctx context.Context, code, language string) (*Result, error)
 		}
 	}
 
+	// Compute kill statistics (combat kills credited to candidate).
+	totalKills := 0
+	totalMatches := 0
+	for _, o := range result.Outcomes {
+		if o.Err != nil {
+			continue
+		}
+		totalMatches++
+		// CombatDeaths[player_idx] contains kills credited to that player
+		if o.CandidateSlot < len(o.CombatDeaths) {
+			totalKills += o.CombatDeaths[o.CandidateSlot]
+		}
+	}
+	result.TotalKills = totalKills
+	result.TotalMatches = totalMatches
+	if totalMatches > 0 {
+		result.KillRate = float64(totalKills) / float64(totalMatches)
+	}
+
 	// Build ordered win-rate vector for PSRO (one entry per distinct opponent).
 	seen := make(map[string]bool)
 	for _, o := range result.Outcomes {
@@ -312,6 +337,7 @@ func (a *Arena) runMatch(ctx context.Context, candidateURL string, opp BotRecord
 	outcome.Winner = res.Winner
 	outcome.Scores = res.Scores
 	outcome.Turns = res.Turns
+	outcome.CombatDeaths = res.CombatDeaths
 	return outcome
 }
 
diff --git a/cmd/acb-evolver/internal/prompt/builder.go b/cmd/acb-evolver/internal/prompt/builder.go
index 06d64c3..d1c1869 100644
--- a/cmd/acb-evolver/internal/prompt/builder.go
+++ b/cmd/acb-evolver/internal/prompt/builder.go
@@ -150,6 +150,13 @@ func writeSystemContext(sb *strings.Builder, targetLang string) {
 	sb.WriteString("- Collect energy tiles (uncontested adjacent bots only) to gain energy.\n")
 	sb.WriteString("- Win by: sole survivor, dominance (≥80% bots for 100 turns), or highest score at turn 500.\n")
 	sb.WriteString("- Vision radius²=49 (~7 tiles). Fog of war: you only see tiles within vision of your bots.\n\n")
+
+	sb.WriteString("## Fitness Function\n")
+	sb.WriteString("- Your evolved bot will be evaluated using fitness = 0.7 * win_rate + 0.3 * kill_rate\n")
+	sb.WriteString("- Win rate: percentage of matches won\n")
+	sb.WriteString("- Kill rate: average kills per match (combat aggression is rewarded)\n")
+	sb.WriteString("- This means: combat kills and eliminations are valuable, not just resource foraging\n")
+	sb.WriteString("- Aggressive combat strategies that secure kills while winning are strongly favored\n\n")
 	sb.WriteString("## HTTP Protocol\n")
 	sb.WriteString("- Your bot is an HTTP server listening on port 8080.\n")
 	sb.WriteString("- Engine POSTs game state (JSON) to /turn each turn. You have 3 seconds to respond.\n")
diff --git a/cmd/acb-evolver/run.go b/cmd/acb-evolver/run.go
index 3b7fe8c..cd9bcad 100644
--- a/cmd/acb-evolver/run.go
+++ b/cmd/acb-evolver/run.go
@@ -598,9 +598,14 @@ func runCycle(ctx context.Context, db *sql.DB, store *evolverdb.Store,
 		return false, fmt.Errorf("arena: %w", err)
 	}
 
-	// Compute fitness (overall win rate)
+	// Compute fitness (weighted combination of win rate and kill rate)
 	wr := arena.ComputeFromResult(arenaResult)
-	fitness := wr.Rate
+	winRate := wr.Rate
+	killRate := arenaResult.KillRate
+
+	// Fitness = 70% win rate + 30% kill rate
+	// This encourages combat aggression while still rewarding winning
+	fitness := 0.7*winRate + 0.3*killRate
 
 	// Get behavior vector
 	var behaviorVec []float64
@@ -614,8 +619,9 @@ func runCycle(ctx context.Context, db *sql.DB, store *evolverdb.Store,
 	store.UpdateFitness(ctx, programID, fitness, behaviorVec)
 
 	if verbose {
-		log.Printf("  Arena result: %d W / %d L / %d D / %d err  win rate=%.3f",
-			arenaResult.Wins, arenaResult.Losses, arenaResult.Draws, arenaResult.Errors, fitness)
+		log.Printf("  Arena result: %d W / %d L / %d D / %d err  win_rate=%.3f  kill_rate=%.3f (%d kills/%d matches)  fitness=%.3f",
+			arenaResult.Wins, arenaResult.Losses, arenaResult.Draws, arenaResult.Errors,
+			winRate, killRate, arenaResult.TotalKills, arenaResult.TotalMatches, fitness)
 	}
 
 	// 7. Load MAP-Elites grid and apply promotion gate